THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 

LOS  ANGELES 


-  last  date 


SOUTHERN  BRANCH, 

UNIVERSITY  OF  CALIFORNIA, 

LIBRARY, 

lLOS  ANGELES,  CALIF. 


LIBRARY 

OF 

PRACTICAL  ELECTRICITY 


VOLUME  V 


PUBLISHERS     OF     BOOKS      f  O  R_, 

Electrical  World  v  Engineering  News -Record 
Power  v  Engineering  and  Mining  Journal-Press 
Chemical  and  Metallurgical  Engineering 
Electric  Railway  Journal  v  Coal  Age 
American  Machinist  ^  Ingenieria  Intemacional 
Electrical  Merchandising  v  BusTransportation 
Journal  of  Electricity  and  Western  Industry 
Industrial  Engineer 


CENTKAL  STATIONS 


BY 
TERRELL  CROFT 

CONSULTING  ELECTRICAL  ENGINEER 


FIRST  EDITION 
TENTH  IMPRESSION 


McGRAW-HILL  BOOK  COMPANY,  INC. 

NEW  YORK:    370  SEVENTH  .  AVENUE 
LONDON:    6  «fe  8  BOUVERIE  ST.,  E.  C.  4 

50279 


COPYRIGHT,  1917,  BY  THE 
MCGRAW-HILL  BOOK  COMPANY,  INC. 

PRINTED  I N  THE  UNITED  STATES  OF  AMERICA 


n 


PREFACE 

A  book  on  central  stations  must  necessarily  relate  to  the 
generation,  transmission  and  distribution  of  electrical  energy. 
Therefore,  the  present  volume  deals  with  these  subjects  —  but 
it  has  not  been  found  expedient  to  classify  the  material  into 
these  three  divisions.  However,  consideration  of  each  of  the 
eighteen  different  section  headings  will  render  it  apparent 
that  every  section  does  pertain  to  either  generation,  trans- 
mission, or  distribution.  Throughout,  the  treatment  is  such 
that  the  gist  of  all  which  is  recorded  can  be  grasped  by  a 
reader  of  modest  mathematical  attainments.  In  other  words, 
a  practical  man  can  study  the  book  understandingly. 

In  the  opening  chapters,  after  certain  elements  which  occur 
in  all  electrical-energy-distribution  systems  have  been  defined, 
the  different  factors  or  coefficients  which  are  utilized  frequently 
in  central  station  practice  are  discussed  rather  exhaustively. 
Among  these  are  load  factor,  demand  factor,  diversity  factor, 
plant  factor  and  the  like.  Their  application  in  the  design  and 
operation  of  central-station  systems  is  explained  and  in  many 
instances  illustrated  by  numerical  examples.  Next  the  typical 
load  curves  or  graphs  which  are  encountered  in  everyday 
work  are  considered.  Then  the  principles  of  circuit  design  — 
both  alternating  and  direct-current  —  are  given  attention; 
examples  showing  how  circuits  are  computed  in  practice  are 
worked  out  in  detail.  Following  the  fundamentals  just  re- 
cited, the  elements  of  transmission  and  distribution  are 
examined.  Transmission  lines,  substations  and  lightning- 
protection  equipment  are  treated. 

Final  chapters  in  the  book  concern  electrical-energy  generat- 
ing stations  and  the  equipment  thereof.  Thus  automatic 
voltage  regulators,  switchboards  and  switchgear  are  treated. 
The  three  different  types  of  prime  movers,  (1)  steam,  (2) 
internal  combustion  engine,  and  (3)  hydraulic,  and  the  adapt- 
ability of  each  of  these  three  different  types  to  certain  con- 


vi  PREFACE 

ditions,  are  studied.  Reactors  and  transformers  are  con- 
sidered briefly.  Numerous  illustrations,  showing  modern 
central-station  practice,  are  given  in  a  number  of  the  chapters 
of  the  book  in  connection  with  the  text. 

Although  the  proofs  have  been  read  and  checked  very  care- 
fully by  a  number  of  persons,  it  is  possible  that  there  remain 
some  undiscovered  errors.  Readers  will  confer  a  great  favor 
by  advising  the  author  of  any  such  which  may  be  revealed. 
Suggestions  for  the  enlargement  or  improvement  of  future 
editions  of  the  book  will  be  greatly  appreciated. 

TERRELL  CROFT. 

33  AMHERST  AVENUE,  UNIVERSITY  CITY, 

SAINT  Louis,  MISSOURI, 

October,  1917. 


ACKNOWLEDGMENTS 

The  author  desires  to  acknowledge  the  assistance  which  has 
been  rendered  by  concerns  and  individuals  in  the  preparation 
of  this  book.  Considerable  of  the  material  is  from  articles 
by  the  author,  which  originally  appeared  in  some  of  the  tech- 
nical periodicals,  among  which  are: — Power,  Electrical  Review 
and  Western  Electrician,  National  Electrical  Contractor,  and 
Power  Plant  Engineering.  Among  the  concerns  which  co- 
operated with  the  author  in  supplying  data  and  copy  for 
illustrations  are : — The  General  Electric  Company,  Schenectady, 
N.  Y. ;  The  Westinghouse  Electric  and  Manufacturing  Company, 
East  Pittsburgh,  Pa. ;  The  Fitz  Water-wheel  Company,  Hanover, 
Pa.;  The  Skinner  Engine  Company,  Erie,  Pa.;  The  Ames  Iron 
Works,  Oswego,  N.  Y. ;  The  Electric  Service  Supplies  Company, 
Chicago,  111.;  and  The  Allis-Chalmers  Company,  Milwaukee, 
Wis.  Certain  of  the  illustrations  are  based  on  those  in  con- 
tributions published  in  Electrical  World,  Electrical  Review  and 
Western  Electrician,  Coal  Age,  Engineering  News  and  Practical 
Engineer.  Several  of  the  tables  of  demand  factors  and  diver- 
sity factors  are  from  the  valuable  book,  "Central  Station 
Distributing  Systems,"  by  Gear  and  Williams  (D.  Van  Nos- 
trand  Company),  who  have  made  exhaustive  studies  concern- 
ing central-station-distribution  characteristics. 

S.  C.  Wagner,  Superintendent  of  Distribution  of  the  Electric 
Company  of  Missouri,  read  the  galley  and  page  proofs,  called 
attention  to  a  number  of  errors  and  suggested  numerous 
improvements. 

Specific  acknowledgments  have  been  made  in  a  number  of 
instances  throughout  the  book.  If  any  has  been  omitted,  it 
has  been  through  oversight  and,  if  brought  to  the  author's 
attention,  will  be  incorporated  in  the  next  edition. 


vii 


CONTENTS 

PAGE 

PREFACE  v 

SECTION  1 
DISTRIBUTION-SYSTEM  NOMENCLATURE 1 

SECTION  2 
DISTRIBUTION  Loss  AND  DISTRIBUTION  Loss  FACTORS 7 

SECTION  3 
MAXIMUM  DEMAND  AND  DEMAND  FACTORS 15 

SECTION  4 
DIVERSITY  AND  DIVERSITY  FACTORS 53 

SECTION  5 
LOAD  FACTOR,  PLANT  FACTOR  AND  CONNECTED-LOAD  FACTOR  .    .     73 

SECTION  6 
LOAD  GRAPHS  AND  THEIR  SIGNIFICANCE 101 

SECTION  7 
GENERAL  PRINCIPLES  OF  CIRCUIT  DESIGN 118 

SECTION  8 
CALCULATION  AND  DESIGN  OF  DIRECT-CURRENT  CIRCUITS  .    .    .    .132 

SECTION  9 
CALCULATION  AND  DESIGN  OF  ALTERNATING-CURRENT  CIRCUITS  .  140 

SECTION  10 

TRANSMISSION  AND  DISTRIBUTION  OF  ELECTRICAL  ENERGY.   .    .    .   173 
SECTION  11 

LIGHTING  PROTECTION  APPARATUS 195 

SECTION  12 
AUTOMATIC  VOLTAGE  REGULATORS.  .  .  219 


x  CONTENTS 

SECTION  13 

PAQB 
SWITCHBOARDS  AND  SWITCHGEAR 229 

SECTION  14 

CHARACTERISTICS  OF  ELECTRIC  GENERATING  STATIONS 259 

SECTION  15 

ADAPTABILITY  OP    STEAM,   INTERNAL   COMBUSTION   ENGINE    AND 
HYDRAULIC  PRIME  MOVERS 281 

SECTION  16 
STEAM  ELECTRICAL-ENERGY-GENERATINQ  STATIONS 287 

SECTION  17 
INTERNAL  COMBUSTION  ENGINE  STATIONS 306 

SECTION  18 

HYDRO-ELECTRIC  STATIONS 310 

INDEX  .  .  325 


CENTRAL  STATIONS 

SECTION  1 
DISTRIBUTION-SYSTEM  NOMENCLATURE 

1.  Considerable  Confusion  Exists  as  to  the  precise  meanings 
of  the  terms  which  are  used  to  designate  the  different  compo- 
nents (Fig.  1)  of  an  electrical -energy-distribution  system. 
In  the  following  paragraphs  definitions  are  given  for  some  of 
the  terms  most  commonly  used.  These  definitions  are,  it  is 
believed,  in  line  with  the  generally  accepted  meanings  of  the 
words  involved.  Fig.  2  shows  diagrammatically  the  impor- 
tant elements  of  a  distribution  system. 


,Hya/ro-aecfric 
•  Generating  Station 


Distributing  Cen 


]_^-8uMw 

!     Distributing  System 
K— — ' 40 Miles * ->J         ln  Town 

FIQ.  1. — Transmission  line  and  distributing  system. 

2.  A  Transmission  Line  comprises  the  arrangement  of  aerial 
conductors  over  which  electrical  energy  is  transmitted  from 
a  generating  station  to  a  sub-station.     In  general,  the  distin- 
guishing characteristics  of  transmission  lines  are  that  they 
operate  at  relatively  high  voltages  and  extend  for  long  dis- 
tances.    At  B  in  Fig.  3  is  shown  the  transmission  line. 

EXAMPLE. — A  pole  line  between  a  city,  industrial  plant  or  building, 
and  a  distant  generating  station  is  a  transmission  line. 

3.  A  Tie  Line  is  a  set  of  aerial  conductors  used  to  intercon- 
nect two  sub-stations,  transmission  lines  or  any  other  lines. 

l 


CENTRAL  STATIONS 


[ART.  4 


•Meters 


m} 


A  tie  line  may  also  operate  at  a  high  voltage  and  extend  for  a 
long  distance  but  is  distinguished  from  a  transmission  line  in 
that  neither  of  the  ends  of  a  tie  line  ordinarily  originates  in  a 
generating  station. 

EXAMPLE. — A  line  connveying  energy  from  one  town  to  another  in 
neither  of  which  there  is  a  generating  station  but  both  of  which  are 
supplied  by  some  circuit  other  than  the  connecting  line,  is  a  tie  line. 

4.  A  Transmission  System  is  one  over  which  electrical 
energy  is  transmitted  for  a 
considerable  distance  from  a 
generating  station,  at  rela- 
tively high  voltage,  to  a  dis- 
tributing system  or  to  dis- 
tributing systems.  A  trans- 
mission system  comprises  the 
conductors  and  the  structures 
which  support  them. 

5.  A  Distributing  System  is 
one  from  which  electrical  en- 
ergy is  distributed  to  con- 
sumers or  to  receiving  appa- 
ratus. A  distributing  system 
consists  of  feeders,  mains,  ser- 
vices, etc.,  as  shown  in  the 
illustrations.  All  of  the 
wiring  in  a  town,  industrial 
iGenerahncfSw/on  plant  or  community  between 
the  sub-station  (or  the  gener- 
ating station  if  the  energy  is 
generated  locally)  and  the 
service  switches  for  buildings 

or  consumers  constitutes  a  distributing  system.  As  a  rule  a 
distributing  system  operates  at  a  lower  voltage  than  does  a 
transmission  system. 

NOTE. — It  is  very  difficult  to  distinguish  between  a  transmission  and  a 
distributing  system,  because:*  "In  any  large  system  the  functions  of 
transmission  and  distribution  merge  into  one  another  because  the  prin- 

•  P.  H.  Thomas. 


FIQ.  2. — The  elements  of  a  transmis- 
sion and  distribution  system. 


SEC.  1]       DISTRIBUTION-SYSTEM  NOMENCLATURE  3 

cipal  consumers  will  ordinarily  be  many  miles  apart.  Furthermore, 
there  usually  are  several  sources  of  energy  feeding  into  the  system  at 
different  locations.  The  transmission  and  distribution  systems  then 
resolve  themselves  into  a  network  of  high-tension  lines  to  which  are 
connected  consumers  and  generators  at  certain  locations. 

6.  A  Feeder  or  Feeder  Circuit  (Fig.  3)  is  the  set  of  con- 
ductors in  a  distributing  system  extending  from   the   ori- 
ginal source  of  energy  in  the  installation  to  a  distributing  center 
and  having  no  other  circuits  connected  to  it  between  the 
source  and  the  center.     The  source  may  be  a  generating  or 
substation  or  a  service.     Feeders  are  indicated  by  the  letter 
D  in  Fig.  3. 

7.  A  Sub -feeder  is  an  extension  of  a  feeder  from  one  distri- 
bution center  to  another  and  having  no  other  circuit  connected 
to  it  between  the  two  distribution  centers.     A  sub-feeder  is  a 
sort  of  a  tie  line. 

8.  A  Main  (E  and  G,  Fig.  3)  is  any  supply  circuit  to  which 
other  consuming  circuits — sub-mains,  branches  or  services — 
are  connected  through  automatic  cut-outs — fuses  or  circuit- 
breakers — at  different  points  along  its  length.     Where  a  main 
is  supplied  by  a  feeder  the  main  is  frequently  of  smaller  wire 
than  the  feeder  which  serves  it.     An  energy  utilizing  device 
is  never  connected  directly  to  a  main,  a  cut-out  always  being 
interposed  between  the  device  and  the  main. 

9.  A  Sub-main  (Ei,  Fig.  3)  is  a  subsidiary  main  fed,  through 
a  cut-out,  from  a  main  or  another  sub-main  and  to  which 
branch  circuits  or  services  are  connected  through  cut-outs. 

10.  A  Service  (or  a  service  connection,  H,  Fig.  3)  is  the  set 
of  conductors  constituting  an  underground  or  overhead  con- 
nection between  conductors  (usually  belonging  to  a  public 
service  corporation)  in  a  thoroughfare — street — and  the  con- 
ductors of  an  interior  or  isolated  wiring  system.     A  "service" 
provides  a  path  over  which  electrical  energy  is  delivered  to  the 
consumers. 

11.  A  Branch  or  Branch  Circuit  is  the  set  of  conductors, 
feeding  through  an  automatic  cut-out  (from  a  distribution 
center,  main  or  sub-main)  to  which  one  or  more  energy  utilizing 
devices  are  connected  directly,  that  is,  without  the  interposi- 
tion of  additional  cut-outs.    The  only  cut-out  associated  with 


[ART.  11 


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H 

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£ 

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-•Disconnecting  Switch 
?    ^Primary  Mains 


Distribution  Center--'       Branches-' 
Fro.  3. — Pictorial  diagram  indicating  the  elements 


SEC.  1]       DISTRIBUTION-SYSTEM  NOMENCLATURE  5 


Transmission  Line*' 


-Stack 

\    Generating 

\    / 'Station 


of  an  electrical-energy  distribution  system. 


6  CENTRAL  STATIONS  [ART.  12 

a  branch  is  the  one  through  which  the  branch  is  fed  at  the  main, 
sub-main  or  distribution  center. 

12.  A  Tap  or  Tap  Circuit  (Fig.  3)  is  a  circuit,  serving  a  single 
energy-utilizing  device,  connected  directly  to  a  branch  without 
the  interposition  of  a  cut-out. 

13.  A  Distributing  or  Distribution  Center  (Fig.  3)  in  an 
electrical  energy  distribution  system  is  the  location  at  which  a 
feeder  or  sub-feeder  connects  to  the  subordinate  circuits  which 
it  serves.     The  switches  and  automatic  cut-outs  for  the  con- 
trol and  protection  of  the  sub-circuits  are  usually  grouped  at 
the  distribution  center.     In  interior-wiring  parlance,  a  dis- 
tribution center  is  an  arrangement  or  group  of  fittings  whereby 
two  or  more  minor  circuits  are  connected  at  a  common  point  to 
another,  larger  circuit.     A  panel  box  is  one  form  of  a  distribu- 
tion center. 

14.  The  Nomenclature  of  Interior-wiring-system  Circuit 
Elements  is  similar  to  that  for  an  outside  distribution  system.* 
The  terms  feeder,  sub-feeder,  main,  sub-main,  branch,  tap  and 
distribution  center  are  defined  diagrammatical  ly  in  the  wiring 
layout  shown  in  the  lower  part  of  the  center  of  Fig.  3.     From 
inspection  it  is  evident  that  these  feeder-system  elements  can 
also  be  defined  in  essentially  the  same  words  as  recited  for  the 
outside  distribution-system  elements  recited  above. 

*  See  the  author's  AMERICAN  ELECTRICIANS'  HANDBOOK,  p.  109,  for  detail. 


SECTION  2 

DISTRIBUTION  LOSS  AND  DISTRIBUTION- 
LOSS  FACTORS 

15.  Distribution  Losses  are  those  losses  of  electrical  energy 
which  occur  in  a  central-station  system  between  the  station 
feeding  the  community  and  the  receiving  devices  on  the  cus- 
tomers' premises.  These  same  energy  losses  are  sometimes 
referred  to  as  "energy  lost  or  unaccounted  for"  or  "kilowatt- 
hours  lost  or  unaccounted  for. "  In  every  electrical -energy-dis- 
tribution plant,  the  total  kilowatt-hours  delivered  to  the  dis- 
tribution-system lines,  as  recorded  by  the  station  totalizing 
watt-hour  meter,  during  a  given  interval  of  time  will  always 
be  greater  than  the  sum  of  the  kilowatt-hours  registered  on  all 
of  the  consumers'  meters  or  similarly  accounted  for  during 
the  same  interval  of  time.  The  difference  between  the  energy 
thus  delivered  to  the  distributing  system  and  that  accounted 
for,  represents  energy-distribution  losses  or  "energy  unac- 
counted for. " 

EXAMPLE. — In  a  certain  Middle-Western  town  of  6,000  people  the 
central  station  operates  a  2,200-volt-primary,  110-220-volt,  three-wire- 
secondary  system.  There  are  a  large  number  of  small  transformers  and 
there  is  considerable  leakage  where  the  primary-line  wires  come  into 
contact  with  the  limbs  of  trees.  During  the  year  of  1914  the  plant 
generated  and  delivered  to  the  lines  348,000  kw-hr.  of  energy.  During 
the  same  year  the  energy  recorded  on  all  of  the  customers'  watt-hour 
meters  and  otherwise  accounted  for  was  only  251,000  kw.-hr.  Hence, 
in  this  instance  the  energy  distribution  loss  was:  348,000  —  251,000  = 
97,000  kw.-hr. 

EXAMPLE. — In  a  town  of  less  than  1,000  inhabitants  in  Iowa  where  a 
110-220-volt  alternating-current,  three-wire  system  (without  trans- 
formers) is  used  for  distribution,  the  central  station  delivered  in  1913  to 
the  lines  25,000  kw.-hr.  The  customers'  meters,  for  the  same  interval, 
recorded  23,000  kw.-hr.  Hence,  for  this  plant  and  this  year,  the  dis- 
tribution loss  was:  25,000  -  23,000  =  2,000  kw.-hr. 
7 


8  CENTRAL  STATIONS  [ART.  16 

EXAMPLE. — For  the  year  ending  June  30,  1915*  The  Pacific  Power 
and  Light  Company,  which  operates  in  the  States  of  Oregon  and  southern 
Washington,  generated  or  bought  45,473,923  kw.-hr.  of  energy.  For 
the  same  period,  the  energy  delivered  to  customers  or  otherwise  ac- 
counted for  was  only  37,746,854  kw.-hr.  Hence,  the  distribution  loss 
for  that  year  was  7,727,069  kw.-hr. 

16.  Distribution  Loss  Includes  all  of  the  energy  which  is  not 
delivered  to  customers  or  otherwise  accounted  for  and  is  made 
up  of,  or  comprises,  a  number  of  secondary  losses  which  may  be 
enumerated  thus: 

(a)  Line  loss  (I2  X  R  losses)  in  the  line  conductors,  feeders, 
mains  and  services. 

(b)  Leakage    loss    (due    to   insufficient   insulation,    grounds 
against  trees  and  the  like}. 

(c)  Transformer    loss   (due    to    the  iron  and  copper  losses 
in   the   transformers.     These  occur  only  in  alternating-current 
plants). 

(d)  Meter  loss  (due  to  slow  meters  and  to  the  electrical  losses 
within  the  meters). 

(e)  Stolen-energy  loss  (occasioned  by  "theft  of  current"). 

17.  The  Line  Loss — that  is  the  kilowatt-hours  energy  lost 
in  the  line  conductors — can  be  readily  computed  if  the  resist- 
ance of  the  line  and  the  current  in  it  is  known,  because:  Watts 
line  loss  =  (line  current  in  amp.)2  X  (resistance  of  line  in  ohms) . 
Then,  if  the  watts  power  loss  thus  obtained  be  multiplied  by 
the  number  of  hours  during  which  the  current  flows,  the  kilo- 
watt-hour energy  line  loss  will  be  the  result      Since  the  current 
in  a  distribution  line  is  seldom  constant,  it  is  necessary  to  recog- 
nize this  condition  in  computing  energy  line  loss.     The  line 
current  will  vary  from  hour  to  hour  and  month  to  month. 
However,  the  approximate  loss  can  be  readily  calculated  if  24- 
hr,  load  curves  for  four  typical  months  of  the  year — say 
March,  June,  September  and  December — are  available.     The 
load  factor  of  the  plant  has,  obviously,  a  bearing  on  its  line 
loss.     The  process  is  a  trifle  tedious  and  a  detailed  description 
of  it  would  be  out  of  place  here,  f 

•  Electrical  Review,  Nov.  13,  1915:  p.  901. 

t  For  a  complete  discussion  of  the  method  se«  Gear  and  Williams,  ELECTRIC  CENTRAL 
STATION  DISTRIBUTING  SYSTEMS,  p.  274. 


SEC.  2]  DISTRIBUTION-LOSS  FACTORS  9 

NOTE. — The  line  loss  may  be  either  a  large  or  a  small  proportion  of  the 
distribution  loss,  depending  on  whether  large  or  the  smallest  feasible 
conductors  are  used  for  the  distribution  lines.  If  the  designer  provides 
excessively  large  conductors,  the  line  loss  will  then  be  very  small. 
However,  as  plants  are  usually  designed,  the  line  loss  is  relatively  small. 
The  matter  of  the  economic  conductor  design  is  treated  in  detail  in  various 
standard  works. 

18.  The  Leakage  Loss  will  be  determined  wholly  by  the 
thoroughness  with  which  the  line  was  originally  constructed 
and  by  the  effectiveness  of  its  maintenance.     If  the  conductors 
are  supported  on  insulators  of  proper  design  and  material,  if 
the  conductors  are  held  away  from  tree  branches  on  insulators 
and  if  the  trees  through  which  the  line  passes  are  well  trimmed, 
the  leakage  loss  will  be  very  small.     There  is  no  practicable 
method  of  computing  the  leakage  losses.     They  can  be  deter- 
mined by  test,  but  this  is  usually  impracticable,  because  it  in- 
volves the  simultaneous  opening  of  every  consumers'  service 
switch. 

19.  Transformer  Losses  are  subdivided  into  copper  losses 
and  core  losses.     The  copper  loss  of  each  transformer  varies 
with  its  load  and  with  no  load  on  a  transformer  its  copper  loss 
is  very  small.     The  iron  loss  is  practically  constant  so  long  as 
normal  line  voltage  is  impressed  across  the  primary  terminals 
of  the  transformer — whether  the  secondary  is  loaded  or  not. 
The  transformer  loss  is  likely  to  be  a  large  proportion  of  the 
total  distribution  loss,  particularly  where  the  load  is  almost 
wholly  lighting  and  the  plant  operates  24  hr.  a  day — because 
the  core  loss  is  "building  up"  every  hour  that  the  transformer 
is  connected  to  the  line.     Small  underloaded  transformers  are 
a  source  of  excessive  loss.     Hence,  the  capacities  of  trans- 
formers should  be  carefully  determined  so  that,  in  general,  a 
few  large  fully-loaded  transformers  will  be  used  rather  than 
many  small  underloaded  ones. 

NOTE. — The  transformer  loss  of  a  system  may  be  computed  approxi- 
mately, if  the  ratings  of  the  different  transformers,  theii  efficiencies  and 
the  loads  and  duration  thereof  which  are  imposed  on  them  are  known. 
The  process  is  tedious  but  feasible. 

19a.  Meter  Losses  are  relatively  small  although  they  may 
in  the  aggregate  be,  contrary  to  the  generally  accepted  opin- 


11) 


CENTRAL  STATIONS 


[ART.  20 


?! 

**! 


s%* 

'•3.S? 


1^ 

jL\ 

2 


^ 

*K 

% 


ion,  greater  than  the  line  losses.     The  power  loss  in  the  shunt 
or  voltage  coils  of  watt-hour  meters 
will,  probably,  range  between  1  watt 
and  4  watts.     Meters  of  the  older 
designs  appear  to  have  the  greater 
losses.     The  power  loss  in  the  volt- 
5?  age  coil  of  an  average  modern  meter 
§  is,  likely,  in  the  neighborhood  of  2.4 
J  watts.     The  power  loss  in  the  cur- 
•3  rent  coil  is  very  small,  almost  neg- 
i-  ligible,  even  when  it  does  occur — 
^  and  it  occurs  only  when  current  to 
1  serve  a  load  is  passing  through  the 
y  meter.     But  the  loss  in  the  voltage 
coil  occurs  continuously — as  long  as 
'•§  voltage  is  impressed  on  the  meter. 
|  The  total  aggregate  energy  loss  thus 
"3  involved  maybe  relatively  consider- 
Ti  able  as  indicated  by  the  following 
*  example. 

.2  EXAMPLE. — If  the  power  loss  in  the 
|  voltage  coil  of  a  watt-hour  meter  is  1.5 
J  watts  and  the  meter  is  connected  to  the 
"a  lines  of  a  plant  giving  24-hr,  service,  what 
•J  will  be  the  energy  loss  in  this  coil  in  a 
|  year?  SOLUTION.— There  are  8,760  hr. 
-g  in  a  year.  Hence,  the  energy  loss  hi  a 
'•*  year  would  be:  1.5  watts  X  8,760  hr.  = 
|  13,140  watt-hr.  =  13.1  kw.-hr. 

J  20.  Stolen-energy  Loss  is,  obvi- 
J  ously,  difficult  of  determination. 
|  Whether  or  not  it  assumes  material 
1  values  depends  largely  on  the  policy 
•*  and  vigilance  of  the  concern  which 


2  is  giving  service.  However,  in  any 
case,  it  is  likely  to  be  but  a  small 
proportion  of  the  total  distribution 
loss,  but  it  is,  probably,  larger  than 
most  people  imagine. 


SEC.  2]  DISTRIBUTION-LOSS  FACTORS  11 

21.  A  Specific  Numerical  Illustration  of  Distribution  Loss  and 
the  Segregation  Thereof  is  given  graphically  in  Fig.  4.  The 
values  there  shown,  and  also  recited  in  Table  22,  were  computed 
from  actual  operating  data,  for  the  year  1913,  of  a  central- 
station  plant  in  a  city  of  about  6,500  inhabitants  in  Missouri. 
The  60-cycle,  two-phase,  distribution  system  under  considera- 
tion comprises  2,400  volts  primary  and  a  110-220-volts,  three- 
wire  secondary.  While  the  distribution  loss  indicated  in  Fig. 
4  is  greater  than  it  would  be  in  a  well-designed  distribution 
plant,  there  are,  probably,  many  small  central-station  systems 
operating  in  the  United  States  which  could  not  show  a  much 
better  performance. 

EXPLANATION. — During  the  year  1913,  250,000  kw.-hr.  of  electrical 
energy  were  delivered  to  the  customers  of  this  central  station.  During 
this  same  interval,  there  were  supplied  to  the  distribution  system  350,000 
kw.-hr.  Hence,  the  distribution  loss  was:  350,000  —  250,000  kw.-hr.  = 
100,000  kw.-hr.,  during  this  year.  It  was  estimated,  on  the  basis  of 
careful  calculations,  that  the  annual  line  transformer,  and  meter  losses 
were  about  as  shown  in  Table  22.  No  specific  estimates  of  leakage  loss 
or  stolen  energy  were  made,  it  being,  probably,  assumed  that  these  were 
included  in  the  three  loss  items  which  were  estimated. 

The  distribution  plant  included  60  transformers  ranging  in  capacity 
from  2  kva.  to  40  kva.  all  but  four  being  under  20  kva.  in  capacity. 
Probably  many  of  these  transformers  were  of  old  inefficient  types. 
There  were  973  watt-hour  meters  in  the  installation  ranging  in  capacity 
from  200  amp.  to  5  amp.;  all  but  11  were  of  50  amp.  or  less  capacity. 
There  were  704  5-amp.  watt-hour  meters. 

A  consideration  of  these  data  (Fig.  4  and  Table  22)  will  emphasize  the 
importance  of  using  only  high-efficiency  transformers,  if  the  distribution 
loss  is  to  be  maintained  at  a  minimum,  because  in  this  example  the  trans- 
former loss  was  much  the  greatest  of  any  of  the  distribution  losses.  The 
meter  loss  was  greater  by  5,000  kw.-hr.  than  the  line  loss.  These  data 
show  that  the  usually  accepted  notion  that  the  line  loss  is  always  the 
greatest  of  the  distribution  losses  may  not,  by  any  means,  always  be 
right. 

These  data  indicate  the  conditions  obtaining  in  the  small- 
city  distribution  plant  discussed  in  more  detail  in  the  preced- 
ing explanation. 


12  CENTRAL  STATIONS  [ART.  22 

22.  Example  of  Losses  in  the  Distribution  of  Electrical  Energy 


Item 

Annual 
kilowatt-hours 

In  per  cent, 
of  energy  gen. 

In  per  cent, 
of  energy  deliv. 

Line  loss  

10,000 

2  9 

4  0 

Transformer  loss  

75,000 

21  3 

30  0 

Meter  loss 

15  000 

4  3 

6  0 

Total  distribution  loss. 

100000 

28  5 

40  0 

Supplied  to  system. 

350,000 

100  0 

140  0 

Delivered  to  consumers  and  ac- 
counted for.  .  . 

250.000 

71.5 

100.0 

23.  A  Distribution-loss  Factor  is  that  value,  relating  to  some 
particular  system,  expressed  as  a  percentage,  which,  if  the 
energy  delivered  and  accounted  for  be  multiplied  by  it,  will  give 
the  energy  lost  in  distribution.  It  is  the  per  cent,  of  energy 
"sold"  which  is  lost  and  unaccounted  for  in  distribution. 
Therefore : 

/-,  \  ^  •  t  --L  *  kw.-hr.  distribution  loss 

(1)  Distribution-loss  factor  =  -, ^ r~p i i —        ^   ,  ,. 

kw.-hr.  delivered  and  accounted  for 

(2)  Hence,  kw.-hr.  dist.  loss  =  (dist. -loss  factor}  X  (kw.-hr.  del. 
and  ace.  for). 

and 

kw.-hr.   dist.   loss 

(3)  kw.-hr.  del.  and  ace.  for  =      ,.  .   . -=—- 

dist.-loss  factor 

NOTE  that  the  kilowatt-hours  delivered  and  the  kilowatt-hours  lost 
must  both  be  measured  over  the  same  interval  of  time — preferably  over 
an  extended  interval  such  as  6  months  or  a  year. 

EXAMPLE. — In  the  case  of  the  Missouri  small-city  system  (Art.  21) 
the  energy  supplied  to  the  system  hi  the  year  of  1913  was  350,000  kw.-hr. 
During  the  same  year,  the  energy  delivered  to  consumers  was  250,000 
kw.-hr.  What  was  the  distribution-loss  factor  for  this  plant  for  the 
year  1913?  SOLUTION.— The  distribution  loss  was:  350,000  -  250,000 
=  100,000  kw.-hr.  Now  substitute  in  equation  (1):  Dist.-loss  factor  = 
(kw.-hr.  dist.  toss)  +  (kw.-hr.  del  and  ace.  for)  =  100,000  -J-  250,000 
=  0.40  =  40  per  cent.  Hence,  the  distribution-loss  factor  for  this 
plant  for  this  year  was  40  per  cent. 

EXAMPLE.— In  a  certain  small  town  of  500  inhabitants  it  was  estimated 
that  the  total  energy  sold  would  be  16,370  kw.-hr.  annually.  If  it  be 


SEC.  2]  DISTRIBUTION-LOSS  FACTORS  13 

decided  that  the  town  will  be  served  by  an  alternating-current  2,400- 
volts-primary  110-volts-secondary  system  and  that  a  loss  factor  of  20 
per  cent,  be  assumed,  how  many  kilowatt-hour  will  have  to  be  generated 
annually?  SOLUTION. — From  equation  (2):  kw.-hr.  dist.-loss  =  (dist.- 
loss  factor)  X  (kw.-hr.  del.  and  ace.  for)  =  0.20  X  16,370  =  3,274 
kw.-hr.  That  is,  the  annual  distribution  loss  would  be  3,274  kw.-hr. 
Then,  there  would  have  to  be  generated :  16,370  +  3,274  =  19,644fcu>.-fcr. 
Or,  a  more  direct  solution  is:  16,370  X  1.20  =  19,644  kw.-hr. 

EXAMPLE. — In  a  certain  central-station  plant  there  are  789,600  kw.-hr. 
supplied  to  the  distribution  system  annually.  If  the  distribution-loss 
factor  for  this  system  is  assumed  to  be  25  per  cent.,  how  much  energy  is 
delivered  to  customers  and  otherwise  accounted  for  annually  ?  SOLUTION. 
— From  the  preceding  discussion  it  follows  that,  equation  (3):  Kw.-hr. 
dd.  and  ace.  for  —  (kw.-hr.  supplied  to  dist.  system)  -i-  (100  +  dist.- 
loss  factor)  =  789,600  -h  1.25  =  631,000  kw.-hr.  '  Hence,  631,000  kw.- 
hr.  of  energy  would  annually  be  delivered  to  the  customers  of  this 
system. 

24.  Probable  Distribution-loss  Factors,  that  is,  factors  that 
will,  probably,  apply  for  certain  different  conditions  of  service 
are  given  in  Table  26.     These  are  based  on  data  from  a  num- 
ber of  cases  encountered  in  actual  practice  and  are  believed  to 
be  representative.     The  "probable-fair-average-value"  values 
may  be  used  in  making  estimates.     It  should  be  understood, 
as  hereinbefore  suggested,  that  the  distribution-loss  factor  for 
any  certain  installation  will  be  determined  wholly  by  the  char- 
acteristics of  that  system. 

NOTE. — For  a  central  station  where  a  residence-lighting  load  pre- 
dominates and  there  is  little  power  load,  the  distribution-loss  factor 
will  be  much  higher  than  where  the  power  load  is  predominant.  The 
reason  for  this  is  that  with  the  residence-lighting  load,  the  transformer 
loss  will  be  proportionally  large.  It  is  also  true  that  distribution  losses 
will  usually  be  greater,  relatively,  for  a  residence  district  where  the  con- 
sumers are  widely  scattered  than  for  a  district  where  the  loading  is 
dense,  for  the  reason  that  a  few  large  well-loaded  efficient  transformers 
can  be  used  for  serving  the  dense  load  while  the  scattered  load  will, 
probably,  be  fed  by  many  small  underloaded  less-efficient  transformers. 

25.  Line-loss  Factor. — A  line-loss  factor  is  a  value  represent- 
ing the  ratio  of  the  actual  Iz  X  R  energy  loss  in  a  feeder  or 
other  line  circuit  component  during  a  year  to  the  I2  X  R 
energy  loss  that  would  have  occurred  in  that  feeder — or  other 
circuit  component — if  it  had  carried  continuously  the  maxi- 


14 


CENTRAL  STATIONS 


[ART.  26 


mum  load  ever  imposed  on  it  during  the  entire  year.  This 
factor  is  sometimes  indefinitely  referred  to  as  merely  "loss 
factor."  It  is  somewhat  similar  in  derivation  to  load  factor, 
which  is  the  ratio  of  the  average  load  imposed  on  a  station  or 
system  to  the  maximum  load  imposed  on  it.  Ordinarily,  it  is 
impossible  to  obtain  the  "actual"  I2  X  R  energy  loss  in  a 
circuit,  hence  in  computing  line-loss  factor  in  practice,  an 
estimated  approximate  value  is  determined  in  accordance  with 
the  process  referred  to  in  a  preceding  paragraph. 

26.  Approximate  Distribution-loss  Factors. — Values  based 
on  data  from  actual  practice.  These  values  contemplate  all 
of  the  losses  tabulated  in  Art.  16. 


Distribution-loss  factors 

Kind  of  plant  and  general  conditions 

F^      <-^ 

Without  transformers  and  well-de- 
signed; 110-220  three-wire  or  440- 
volt. 

10-220                15 

current 

Well-designed  system  2,200,  2,400  or 
6,600-volt  primary  and  220  or  440- 
volt  secondary  ;  largely  power  load  of 
reasonably  high  load  factor. 

15-2.5                  20 

Alternating 

Well-designed  system;  2,200  or  2,400- 
volt  primary  and  110-200-volt  three- 
wire  secondary;  general  lighting  and 
power  load. 

20-30                  25 

Poorly-designed  system  ;  large  number 
of  small  transformers;  2,200  or  2,400- 
volt  primary  and  110-220-volt  sec- 
ondary; general  lighting  and  power 
load. 

25-45                  35 

15  "3 

f>  P 

Well-designed;  lighting  and  powe: 
load 

5-20                  10 

si 

Poorly  designed;  lighting  and  power 
load. 

10-25                  15 

SECTION  3 
MAXIMUM  DEMAND  AND  DEMAND  FACTORS 

27.  The  Demand  of  an  Installation  or  System*  is  "the  load 
which  is  drawn  from  the  source  of  supply  at  the  receiving 
terminals  averaged  over  a  suitable  and  specified  interval  of 
time.     Demand   is  expressed  in  kilowatts,  kilovolt-amperes, 
amperes  or  other  suitable  units."     It  should  be  noted  that 
"the  load  is  averaged  over  an  interval  of  time."     Hence,  it 
follows  from  this  definition  that  there  is  no  such  thing  as  an 
"instantaneous  demand."    In  other  words,  the  demand  of 
an  installation  is  the  requirement — usually  power  require- 
ment— of  that  installation  averaged  over  a  time  interval. 

28.  The  Average  Demand  of  an  installation  is  the  average 
requirement — usually    power    requirement — of    the    installa- 
tion during  some  specified  interval  of  time  of  considerable 
duration  such  as  a  day,  month  or  year.     Hence,  the  average 
power  demand  of  an  installation  in  kilowatts  for  a  specified 
interval    may   be   obtained   by   dividing   the   kilowatt-hour 
energy  consumed  by  the  installation  during  that  interval  by 
the  number  of  hours  in  the  interval.     This  method  gives  an 
arithmetical  average.     That  is: 

kw.-hr.  during  interval      ,,  ., 

(4)  kw.  average  demand  =  — r — ~ ; —       (kilowatts) 

hours  in  interval 

,_,    r,         .    .   .  kw-hr.  consumed  during  interval  .. 

(5)  Hour  sin  interval  =  —  —  (hours) 

kw.  average  demand 

(6)  Kw.-hr.    consumed     during    interval  =  (kw.-hr.    average 
demand)  X  (hr.  in  interval).  (kilowatts) 

EXAMPLE. — If  the  totalizing  watthour  meter  of  a  central  station 
indicates  that  the  energy  supplied  by  the  station  to  the  system  is  64,723 
kw.-hr.  during  a  certain  24-hr,  day  what  was  the  average  power  demand 
in  kilowatt  for  that  day?  SOLUTION. — From  (4):  few.  average  demand 

•  A.  I.  E.  E.  STANDABDIZA.TION  RULES,  June  28,  1916,  Sec.  57. 
15 


16 


CENTRAL  STATIONS 


[ART.  29 


=  (kw.-hr.  consumed  during  interval)    •*•   (hours  in  interval)    =  64,723 
-5-  24  «=  2,690  kw. 

29.  The  Maximum  Demand  of  an  Installation  or  System 
is*  "the  greatest  of  all  the  demands  which  have  occurred 
during  a  given  period.  It  is  determined  by  measurement 
according  to  specifications  over  a  prescribed  time  interval." 
By  combining  the  definitions  of  "demand"  and  "maximum 
demand"  above  given,  it  is  evident  that  the  maximum  demand 
of  an  installation  is  the  greatest  power  load  occurring  during 
a  certain  relatively  long  period — such  as  a  day,  month  or 
year.  However,  it  is  not  the  greatest  instantaneous  load 


6.30      7-PM.      7.30        8-RM. 


9-RM.        a30        10-PM.      10.30        II-RM.       11.30 


FIG.  5. — Illustrating    one   method  of  obtaining  an  averaged  or  integrated 
maximum  demand  over  a  30-minute  interval. 

during  that  period  but  it  is  the  greatest  average  power  load 
occurring  during  any  of  the  relatively  short  intervals  of  the 
specified  or  selected  duration — such  as  1  min.,  15  min.  or  30 
min. — within  the  period.  The  study  of  the  following  ex- 
ample (Fig.  5)  will  assist  one  in  obtaining  an  understanding 
of  this  statement.  Note  that,  as  will  be  explained  later, 
the  load  during  the  specified  interval  is  not  necessarily 
averaged  arithmetically  because  processes  of  averaging  other 
than  the  arithmetical  are  being  used. 

EXAMPLE. — Fig.  5  shows  the  graph  of  a  load  extending  over  a  5-hr, 
period.  The  maximum  demand  of  this  load  on  a  30-min.-interval 
basis,  is,  as  shown,  277.5  kw.  By  inspection  it  is  appare  it  that  the 
load  is  greater  during  the  30-min.  interval  AB  between  8:30  and  9:00 

•  A.  I.  E.  E.  STANDABDIZATIOK  RULES,  June  28,  1916,  Sec.  58. 


SEC.  3]     MAXIMUM  DEMAND  AND  DEMAND  FACTORS        17 

P.M.  than  it  is  during  any  other  30-min.  interval  in  the  5-hr,  period. 
Then  this  interval,  AB,  is  the  one  over  which  the  demand  must  be 
averaged  to  ascertain  the  30-rnin.  maximum  demand  for  the  load 
suggested. 

By  scaling  the  kilowatt  "instantaneous"  demand  at  ten  equidistant 
points  between  the  8:30  P.M.  ordinate,  AC,  and  the  9:00  P.M.  ordinate 
BD,  ten  values  of  the  kilowatt  demands  at  these  instants  are  obtained. 
The  arithmetical  average  of  these  ten  values  is,  as  shown,  277.5  kw. 
Hence  the  30-min.  maximum,  demand  (averaged  arihtmetically)  of  the 
load  graphed  in  Fig.  5,  is  277.5  kw. 

It  should  be  understood  that  the  method  of  the  above  example  of 
determining  the  arithmetical  average  is  not  absolutely  accurate — but 
it  is  sufficiently  so  for  practical  purposes.  The  accuracy  of  the  method 
depends  on  the  number  of  ordinates  which  are  averaged  and  on  the  pre- 
cision with  which  the  ordinates  are  scaled.  In  general,  the  greater  the 
number  of  ordinates  taken,  the  more  exact  will  be  the  method.  Maxi- 
mum demand  may  also  be  determined  from  a  graph  by  using  a  plani- 
meter  in  much  the  same  way  that  mean  effective  pressure  of  a  steam- 
engine  cylinder  may  be  ascertained  from  an  indicator  diagram. 

30.  The  Unqualified  Term  "Maximum  Demand"  is  In- 
definite; that  is,  a  statement  such  as  "the  maximum  demand 
was  125  kw."  does  not  have  a  specific  meaning.     To  render 
the  statement  of  a  maximum-demand  value  specific,  it  is 
necessary  that  there  be  stated:  (1)  The  duration  of  the  period 
under  consideration;  (2)  the  length  of  the  time  interval  over 
which  the  maximum  demand  was  averaged;  (3)  the  method 
used  in  measuring  or  averaging  the  demand. 

31.  The  Unit  in  Which  Maximum  Demand  Should  be  Ex- 
pressed  will    differ   with   the  problem  under  consideration. 
Since,   as   above  suggested,   demand   may  be  expressed   in 
"kilowatts,  kilovolt-amperes,  amperes  or  other  suitable  units" 
it  follows  that  maximum  demand  may  also  be  expressed  in 
such  units.     What  unit  is  used  in  any  instance  should  be  de- 
termined by  the  purpose  for  which  the  maximum  demand 
observation  was  made  and  by  how  it  was  made.     Maximum- 
demand  values  are  now,  probably,  most  frequently  expressed 
in  kilowatts. 

32.  Demand  Meters  (see  following  illustrations)  are  instru- 
ments which  record  or  indicate  the  maximum  imposed  by  the 
circuit  in  which  they  are  connected.     They  are  arranged  to 
automatically  average  (though  the  average  is  not  necessarily 


18  CENTRAL  STATIONS  [ART.  33 

an  arithmetical  average)  the  power  demand  over  the  selected 
time  interval  for  which  they  are  designed  or  calibrated. 
Several  different  types — the  principles  and  operation  of  some 
of  which  are  briefly  discussed  in  the  succeeding  paragraphs — 
are  available.  Where  such  instruments  can  be  used  the  neces- 
sity of  making  tedious  computations  (for  determining  the 
maximum  demand)  is  eliminated. 

NOTE. — Since,  in  accordance  with  A.  I.  E.  E.  STANDARDIZATION 
RULE  58,  above  quoted,  maximum  demand  may  be  "determined  by 
measurement  according  to  specifications"  several  different  principles 
have,  as  will  be  shown,  been  utilized  for  maximum-demand  meters. 
Furthermore,  the  different  meters  operating  under  these  various  prin- 
ciples will  not  necessarily  indicate  the  same  maximum-demand  value 
when  connected  in  a  circuit  serving  the  same  load. 

33.  The  Reason  Why  It  Is  The  Average  Maximum  Demand 
Over  a  Certain  Definite  Interval  That  is  of  Interest  rather  than 
the  "instantaneous  maximum  demand"  is  this:  Maximum- 
demand  determinations  are  made  most  frequently — if  not 
always — to  enable  one  to  estimate  the  capacity  (cost)  of  the 
electrical  apparatus  or  equipment  required  to  serve  a  certain 
specified  load.  Maximum-demand  values  are,  it  is  true,  im- 
portant factors  in  the  fixing  of  rates  for  electric  service,  but 
the  reason  that  they  are  of  importance  is  because  of  the  bear- 
ing that  they  have  in  establishing  the  capacity  of  the  equip- 
ment, or  indirectly,  the  investment  that  will  be  required  to 
serve  the  consumers.  Now,  practically  all  electrical  apparatus 
will  safely  carry  considerable — possibly  100  per  cent,  or  greater 
— 'Overloads  for  short  periods  without  permanently  adverse 
effects.  Hence,  it  is  not  logical  or  economically  desirable  to 
so  select  the  capacity  of  a  device  that  the  device  will  be  capa- 
ble of  carrying  continuously  the  kilowatt  load  which  will  be 
imposed  on  it  only  momentarily  or  for  very  short  intervals. 
However,  the  device  must,  if  it  is  not  to  be  damaged,  be  capa- 
ble of  safely  and  effectively  carrying  the  maximum  load  which 
will  be  imposed  on  it  for  continued  periods.  For  these  reasons 
"maximum  demand"  is  defined  as  suggested  above. 

EXAMPLE. — Fig.  6  is  the  graph  of  the  power  load  to  be  impressed  on  a 
certain  generator,  showing  how  the  demand  in  this  particular  case  varies 


SEC.  3]     MAXIMUM  DEMAND  AND  DEMAND  FACTORS        19 

with  the  time.  At  A,  B,  C  and  D  are  load  "peaks"  of  respectively  180, 
230,  200,  and  290  kw.  But  all  of  these  peaks  extend  over  relatively 
short  intervals,  possibly  from  5  to  12  min.  Hence,  it  would  not — for  the 
reasons  outlined  above — be  logical  nor  necessary  to  select  the  capacity 
of  the  generator,  which  is  to  be  installed  to  carry  the  load  such  that  it 
could  carry  continuously  the  290  kw.  of  the  peak  D,  the  200-kw.  peak  of 


Fio.  6. — Graph  showing  power  load  to  be  imposed  on  generator. 

C,  the  230-kw.  peak  of  B  or  the  180  peak  of  A.  However,  at  EF  is  a 
demand  of  about  150  kw.  which  continues  for  something  over  30  min.  or  a 
half  hour.  A  demand  extending  over  this  period  would  have  a  very 
appreciable  heating  effect  on  the  generator  serving  it.  Hence,  in  this 
specific  example,  the  consideration  that  should,  probably,  determine  the 


Holes  Punched  in  Paper  Strip-' 

FIG.  7. — Graph  of  current  (amperes)  demand,  made  by  a  graphic  ammeter. 

capacity  of  the  generator  to  be  selected  is  the  30-min.  maximum  demand 
EF,  of  150  kw. — it,  of  course,  being  assumed  that  the  load  conditions  for 
the  4-hr,  period  graphed  in  Fig.  6  are  fairly  typical  of  the  conditions  which 
exist  during  any  period  of  the  generator's  operation. 

EXAMPLE. — In  Fig.  7  is  reproduced  a  (reduced)  portion  of  a  graph  from 
a  graphic  ammeter.     The  30-min. -ampere-maximum  demand  for  the 


20 


CENTRAL  STATIONS 


[ART.  34 


load  which  this  graph  records,  is,  as  shown  at  C,  about  1,100  amp.  The 
peaks  at  A,  B  and  D  represent  very-short-interval  demands  which, 
probably  would  not  be  of  great  consequence  in  choosing  the  proper 
capacity  of  electrical  apparatus  to  serve  a  load  of  the  characteristics 
shown  in  Fig.  7. 

EXAMPLE. — The  piece  of  a  graphic  wattmeter  record  shown  in  Fig.  8 
records  short-period  peaks  at  A,  B  and  E.  However,  the  15-min.  maxi- 
mum demand  of  this  load  is  about  1,000  kw.  as  shown  at  C.  The  30- 
min.  maximum  demand  is  about  900  or  950  kw.,  as  shown  at  D. 

34.  The  Time  Interval  Adopted  in  Practice,  for  maximum- 
demand  determinations,  over  which  the  greatest  demand  is 
averaged*  varies  somewhat  with  the  characteristics  of  the  load 


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FIG.  8. — Graph   of  power  demand  made  by  a  graphic   "curve-drawing" 
wattmeter.  , 

under  observation  and  with  the  policy  of  the  concern  which  is 
measuring  the  demand.  Probably  a  15-min.  interval  is  now 
used  more  generally  than  any  other.  Hence,  when  a  maximum 
demand  in  kilowatts  or  amperes  is  specified  and  no  time  inter- 
val is  stated,  the  chances  are  that  a  15-min.  interval  is  usually 
implied. 

NOTE. — It  appears  that  it  is  the  tendency  of  some  of  the  State  Public 
Service  Commissions  to  advocate  a  30-min.  time  interval  because  thL 
interval — other  things  being  equal — is,  it  is  asserted,  the  more  logical 
for  equipment-capacity  determinations  than  is  a  much  longer  or  a  much 
shorter  one.  For  this  reason  it  is  possible  that  a  30-min.  interval  will, 
in  the  future,  be  adopted  more  generally.  There  are,  likely,  but  very 

*  A.  I.  E.  E.  STANDARDIZATION  RULES,  No.  57. 


SEC.  3]    MAXIMUM  DEMAND  AND  DEMAND  FACTORS       21 

few  cases  where  a  demand  interval  much  longer  than  30  min.  should  be 
adopted. 

35.  The  Time  Intervals  for  Which  Maximum  Demand 
Meters  May  be  Adjusted  are  different  for  the  instruments  of 


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FIG.  9.— Typical     characteristic    curve    of    a   lagged-type    demand    meter 
made  for  a  15-minute  time  interval. 


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FIG.  10. — Typical  characteristic  curve  of  a  lagged-type  demand  meter  made 
for  a  30-minute  time  interval. 

the  different  types.  One  type  of  thermostatic  indicator  is 
adapted  for  only  one  interval — 30  min.  "  Printometer" 
indicators  (see  following  illustrations)  can  be  obtained  which, 
by  providing  a  suitable  contact-making  clock,  will  print  at  the 


22 


CENTRAL  STATIONS 


[ART.  36 


den  of  every  5,  10,  15,  30  or  60-min.  interval.  Watt-hour- 
demand  meters  can  be  obtained  which  will  indicate  the  average 
maximum  demand  over  1,  2,  5, 15  or  30-min.  intervals.  Other, 
induction-type,  demand  meters  can  be  adjusted  for  only  15- 
min.  and  30-min.  intervals.  In  certain  cases  difficulty  has 
been  experienced  in  obtaining  accurate  readings  over  intervals 
much  shorter  than  5  min.,  because,  apparently,  the  maximum- 
demand  mechanisms  are  unable  to  operate  effectively  on  short- 
interval  calibrations.  The  "Wright"  maximum-demand 
meter  which  is,  apparently,  being  superceded  by  other  types, 
has  a  time  interyal  of  approximately  15  min.  Figs.  9  and  10 
show  respectively  typical  characteristic  curves  for  a  15-min. 
and  a  30-min.  demand  meter  of  the  lagged  type. 


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FIQ.  11. — Illustrating  the  effect  of  length  of  time  interval  on  maximum 
demand. 

36.  Examples  Illustrating  the  Effect  of  the  Length  of  the 
Time  Interval  on  the  Resultant  Maximum  Demand  are 

given  in  the  problems  immediately  succeeding.  All  of  them 
refer  to  the  graphs  of  Fig.  11.  They  are  all  imaginary  cases 
and  in  them  the  demand  is  averaged  arithmetically  but  a 
consideration  of  them  will  bring  out  the  facts. 

EXAMPLE  I  (see  Fig.  11). — 15-min.  interval:  Kw.  M.D.  =  kw.-hr. 
+  H  =  (300  lew.  X  0.25  Ar.)  -s-  0.25  hr.  =  75  kw-hr.  0.25  Jar.  =  300  ku>. 
=  15-min. -interval  maximum  demand. 

30-min.  interval:  Kw.  M.D.  =  kw.-hr.  +  H  =  (300  kw.  X  0.50  hr.) 
-5-  0.50  hr.  =  150  kw-hr.  -5-  0.50  hr.  =  300  kw.  =  30-mtn. -interval  maxi- 
mum demand. 

EXAMPLE  II.— 15-min.  interval:  Kw.  M.D.  =  kw.-hr.  -J-  H  =  (400 
kw.  X  0.25  hr.)  •%•  0.25  hr.  =  100  kw.-hr.  •*-  0.25  hr.  =  400  kw.  =  15- 
min.-interval  maximum  demand. 


SEC.  3]    MAXIMUM  DEMAND  AND  DEMAND  FACTORS       23 

30-min.  interval:  Kw.  M.D.  =  kw.-hr.*  H=  [(400  kw.  X  0.25  hr.)  X 
(300  kw.  X  0.25  hr.)]  +  0.50  hr.  =  (100  kw.-hr.  75  kw.-hr.)  +  0.50  hr. 
-  175  kw.-hr.  -r-  0.50  hr.  =  350  kw.  —  30-min. -interval  maximum 
demand. 

EXAMPLE  III.— 15-min.  interval:  Kw.  M.D.  =  kw.-hr.  -i-  H  =  [(350 
fcw>.  +  400  fa*.)  -5-  2]  X  0.25  hr.  -r  0.25  Ar.  =  93.75  fcw.-Ar.  -^  0.25  hr. 
=  375  fcu>.  =  15-min. -interval  maximum  demand. 

30-min.  interval:  Kw.  M.D.  =  Jbw.-ftr.  +  H=  [(300  far.  +  400  kw.) 
-5-  2]  X  0.50  Ar.  -f-  0.50  Ar.  =  175  &u>.-Ar.  -r-  0.50  Ar.  =  350  kw.  =  30- 
min.-interval  maximum  demand. 

EXAMPLE  IV. — A.  15-min.  interval:  Kw.  M.D.  =  kw.-hr.  +  H  = 
(500  kw.  X  0.125  hr.)  +  0.25  X  62.5  kw.-hr.  +  0.25  hr.  =  250  kw.  =  15- 
min.  -interval  maximum  demand. 

A.  30-min.  interval:  Kw.  M.D.  =  kw.-hr.  •*-  H  =  (500  far.  X  0.125) 
0.50  Ar.  =62.5  kw.-hr.  -f-  0.50  hr.  =  125  fcu>.  =  30-rwin.-in<ert;oZ  maximum 
demand. 

EXAMPLE  IV.— B.  15-min.  interval :  Kw.  M.D.  =  Kw.-hr.  -v-  H  =  (350 
kw.  X  0.25  to-.)  -i-  0.25  Ar.  =  87.5  kw.  -5-  0.25  Ar.  =  350  fac.  =  15-min.- 
interval  maximum  demand. 

B.  30-min.  interval:    Kw.  M.D.  =  Kw.-hr.  + H  =  (350  kw.  X  0.25 
hr.  -T-  0.50    Ar.)  87.5   kw'-hr.  -i-  0.50    Ar.  =  175   kw.  =  30-min. -interval 
maximum  demand. 

37.  The  Methods  of  Averaging  the  Load  Over  a  Specified 
Time  Interval  to  obtain  the  maximum  demand  may  be 
divided  into  two  general  classes.  (1)  Arithmetical  average. 
The  maximum  demand  value  obtained  by  this  method  is 
sometimes  called  an  integrated  maximum  demand.  (2) 
Averages — so-called — other  than  the  arithmetical. 

Where  a  maximum  demand  value  is  computed  from  the 
record  of  a  graphic  instrument  (as  for  example  in  Fig.  5)  or 
from  a  series  of  readings  from  an  indicating  instrument,  the 
arithmetical  average  is  the  one  ordinarily  taken.  Further- 
more, the  integrating-type  demand  meters  provide  maximum- 
demand  values  which  may  be  averaged  or  which  have  been 
automatically  averaged  arithmetically  over  the  time  interval 
for  which  the  instrument  is  set. 

Maximum-demand  values  provided  by  certain  of  the  so- 
called  lagged-type  demand  meters  are  averaged  by  the  instru- 
ment, not  arithmetically,  but  ''logarithmically  or  otherwise." 
It  is  unfortunate  that  some  one  method  of  averaging  to  obtain 
the  maximum-demand  values  has  not  been  standardized,  be- 


24  CENTRAL  STATIONS  [ART.  38 

cause,  until  some  specific  one  is,  there  will  be  confusion.  As 
conditions  now  exist,  when  a  maximum-demand  value  is 
stated  it  has  no  particular  significance  unless  supplementary 
information  is  added.  The  reason  why  the  different  methods 
of  averaging  are  encountered,  is  that  the  different  operating 
principles  utilized  in  the  several  types  of  demand  meters  now 
manufactured,  inherently  provide  different  averaging  methods. 

38.  Demand  Meters  of  Different  Types  Will  not  Always 
Give  the  Same  Rating  on  the  Same  Load,  because,  as  above 
suggested,  of  the  principles  that  they  involve  in  "averaging" 
the  demand  over  the  specified  time  interval.     If  a  number  of 
demand  meters  of  the  different  types  are  all  connected  to  the 
same  steady  load  for  a  sufficiently  long  interval  of  time  all  of 
them  will  ultimately  indicate  the  same  maximum-demand 
value.     But  on  fluctuating  loads  the  demand  values  indi- 
cated by  the  instruments  which  operate  on  the   different 
fundamental  principles,  some  of  which  are  briefly  discussed 
below,  may  be  different. 

39.  A  Classification  of  Demand-measuring  Instruments  is 
shown  in  Table  40.     While  this  schedule  is  not  complete  it 
suggests  in  a  general  way  the  underlying  characteristics  of 
some  of  the  devices  most  frequently  used  in  America.     It  is 
not  unlikely  that  at  some  future  time,  because  of  the  "survival 
of  the   fittest"  law  a  much  simpler  classification  will  com- 
prehend   all    of    the    demand-measuring    instruments    used 
commercially. 

40.  Classification  of  Instruments  Used  for  Determining 
Maximum  Demand. — This  is  based  on  the  general  scheme  of 
classification  originally  proposed  by  Mr.  C.  I.  Hall  of  the 
General  Electric  Company.     The  list  of  instruments  tabu- 
lated under  the  heading  "Kinds  of  Instruments  or  Indicators 
Available"  is  not  intended  to  be  complete  as  lines  of  instru- 
ments for  this  service  may  now  be  considered  as  being  in  the 
development  stage. 


SEC.  3]     MAXIMUM  DEMAND  AND  DEMAND  FACTORS        25 


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G.E.  Type  CR. 
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Westinghouse  Graphic. 
Westinghouse  Type  U. 
Bristol. 

Ge  Type  C4. 
G.  E.  Type  CR. 
Bristol. 
Westinghouse  Graphic. 
Westinghouse  Type  U. 
Esterline. 

Westinghouse  Type  Ro. 

G.  E.  Wright  Demand  Indie 
Thermostatic  types. 
Heat  Storage  Types. 

G.E.  Wright  Demand  Indict 
Thermostatic  Types. 
Heat  Storage  Types. 

G.E.  Type  M4  (Indicating) 
G.E.  Type  Pi  (Printometer 
G.E.  Type  G»  (Graphic). 

G.E.  Type  M»  (Indicating) 
G.E.  Type  PJ  (Printometer 
G.E.  Type  Gi  (Graphic). 

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26 


CENTRAL  STATIONS 


[ART.  41 


41.  In  Determining  Maximum  Demand  With  an  Indicating 
Ammeter  it  is  necessary  to  observe  current-intensity  values  at 
equidistant  intervals  on  the  circuit  under  consideration.  Then , 
these  values  may  be  plotted  into  a  graph  similar  to  that  of 
Fig.  5,  wherefrom  the  maximum-demand  value  over  the  se- 
lected time  interval  can  be  computed.  If  it  is  necessary  that 
the  demand  value  be  in  kilowatts  it  is  also  necessary  to  ob- 
serve simultaneous  voltage  values  wherefrom  the  power  can  be 
computed.  On  the  commercial  constant-potential  circuits 
it  is  usually  considered  unnecessary  to  make  simultaneous 
voltage  readings,  inasmuch  as  the  ampere  maximum  demand 
is  usually  the  most  important  for  this  service.  Where  the 
load  is  reasonably  steady  it  is  usually  sufficient  to  take  instru- 


"Conductor.  Current  in  Which  J_  .      ' 
is  being  Measured  '  "  'f/ux'-flBlaritylndfcator  ' 

FIG.  12. — Knopp  test  set  for  determining  current  in  direct-current  conductors. 

ment  readings  every  5  min.  But  with  fluctuating  loads  it  may 
be  necessary  to  take  readings  every  1  min.  or  even  at  shorter 
intervals. 

42.  An  Ammeter  for  Direct-current  Line,  Maximum-de- 
mand Measurements  is  diagrammed  in  Fig.  12.  This  may 
be  utilized  on  either  direct-current  aerial  line  wires  or  on  single- 
conductor  cables  without  its  being  necessary  to  open  the  con- 
ductor under  test.  The  device  may  be  considered  as  comprising 
two  essential  components;  the  exploring  coil,  C,  and  the  meter, 
M.  The  meter,  M,  consists  of  four  elements:  (1)  an  ammeter, 
7,  which  is  actuated  by  the  current  flowing  from  the  battery 
through  the  fine- wire  winding,  W,  of  the  exploring  coil; '(2) 
a  variable  resistor,  R;  (3)  low-voltage  battery,  N;  and  (4)  a  flux 
polarity  indicator,  P. 


SEC.  3]     MAXIMUM  DEMAND  AND  DEMAND  FACTORS        27 

OPERATION. — When  used  to  determine  the  current  in  the  conductor 
the  hinged  section  of  the  exploring  coil  is  opened  and  the  coil  is  placed  so 
as  to  encircle  the  conductor.  The  current,  in  the  conductor  A  under  test, 
creates  a  magnetic  field  in  the  core  of  the  exploring  coil.  This  flux  will, 
provided  the  coil  has  been  placed  over  the  conductor  under  test  in  the 
proper  direction,  cause  the  indicating  magnet,  which  is  mounted  in  a 
small  air  gap,  to  move  in  a  clockwise  direction  establishing  a  contact  at 
B  and  permitting  current  from  the  battery  to  flow  through  the  winding 
of  the  relay  on  the  instrument. 

This  causes  the  flux  polarity  indicator  to  move  toward  the  right,  show- 
ing the  observer  that  the  indicating  magnet  is,  because  of  the  influence 
of  the  flux  due  to  A,  making  a  contact  with  B.  Now,  current  is  per- 
mitted to  flow  from  the  battery  through  the  ammeter  winding  of  M  and 
through  the  fine-wire  winding  on  the  exploring  coil  and  this  current  is 
varied  by  adjusting  the  variable  re- 
sistor, R,  until  the  counter  flux,  due 
to  the  battery,  just  neutralizes  the 
flux  due  to  the  current  in  the  con- 
ductor A, 

When  this  occurs  the  indicating 
magnet  will  swing  out  of  contact 
with  B  and  the  flux-polarity  indi- 
cator will  notify  the  observer  thereof 
and  the  current  read  on  the  ammeter 
scale,  /,  of  the  instrument  M  will  in- 
dicate directly,  or  will  be  propor- 
tional to,  the  current  flowing  in  A. 
The  exploring  coil  may  be  used  at  FlG.  13.— Portable  transformer 
any  reasonable  distance  from  the  open,  cable  having  been  inserted, 
meter  provided  the  number  of  cells 
in  the  battery  is  correspondingly  increased. 

43.  For  Determining  the  Maximum  Demand  in  Alternating- 
current  Lines  the  currents  flowing  in  the  line  can  be  ascer- 
tained with  a  portable  testing  transformer  such  as  that  illus- 
trated in  Figs.  13  and  14.  The  complete  outfit  (Fig.  14) 
consists  of  an  alternating-current  ammeter,  a  split-type 
current  transformer  and  a  suitable  flexible  lead  for  the  inter- 
connection of  the  two  devices.  To  measure  the  current  in  the 
conductor  the  transformer  is  opened  (Fig.  13)  then  closed 
around  the  conductor,  after  which  the  ammeter  (which  has 
been  properly  calibrated  for  use  with  the  transformer  by  its 
manufacturers)  will  indicate  the  current  intensity  in  the  con- 
ductor under  test. 


28  CENTRAL  STATIONS  [ART.  44 

44.  For  Determining  Maximum  Demand  with  a  Graphic 
Wattmeter  or  Ammeter  it  is  merely  necessary  to  examine  the 
graphic  record  produced  by  the  instrument  for  the  period  under 
consideration  and  thereby  find,  by  inspection,  the  greatest 
power  demand  that'  has   occurred  in  any  time  interval   of 
the    prescribed   duration.     Fig.    7   shows    a    typical    record 
from  a  graphic   ammeter   while  Fig.  8    shows  that  from  a 
wattmeter.     These  illustrations  have  been  referred  to  in  pre- 
ceding paragraphs. 

45.  A  Westinghouse  RO  Demand  Meter  is  shown  in  Figs. 
15,  16  and  17.     This  might  be  called  a  lagged-type  in  stru- 


Split-Type 


FIG.  14. — Portable  outfit  for  the  determination  of  current  in  alternating- 
current  lines. 

ment,  inasmuch  as  the  movement  of  its  demand  pointer  is 
retarded  by  the  action  of  an  escapement  wheel  (F,  Fig.  17). 
It  is  made  only  for  alternating-current  service.  The  general 
operating  principle  is  the  same  for  the  single-phase  as  for  the 
polyphase  meter.  These  instruments  are  installed  in  the 
same  manner  as  are  ordinary  watt-hour  meters,  no  additional 
apparatus  or  wiring  being  required.  Each  device  consists  of 
an  induction  watt-hour  meter  to  which  is  added  the  maximum- 
demand-meter  mechanism.  The  maximum  demand,  in  kilo- 
watts, is  indicated  by  the  pointer,  which  moves  over  the  4-in. 
dial.  The  integrating  energy  is  registered  in  kilowatt-hours  on 
the  four-dial  counter. 


SBC.  3]     MAXIMUM  DEMAND  AND  DEMAND  FACTORS        29 


CONSTRUCTION. — An  auxiliary  aluminum  disc  (A,  Fig.  17)  mounted  in 
the  air  gap  of  the  watt-hour  meter  electromagnet  is  provided  to  drive 
the  maximum-demand  pointer  P.  The  speed  of  movement  of  the  pointer 
is,  through  gearing,  retarded  by  the  main  disc.  In  fact,  as  its  maker 
states:  The  mechanism  is  very  similar  to  that  of  an  ordinary  clock.  The 
auxiliary  disc  (A,  Fig.  17)  performs  a  function  somewhat  similar  to  that 
of  a  main  spring,  in  that  it  furnishes  power  for  driving  the  demand  pointer 
and  the  escapement  F,  while  the  rate  of  progression  of  the  demand  pointer, 
P,  is  controlled  by  motion  of  the  main  disc — which  performs  the  function 
of  a  balance  wheel.  It  is,  then,  to  be  observed  that  the  function  of  the 
main  watt-hour-meter  disc  is.  insofar  as  the  demand  mechanism  is  con- 


Mefa/Core, 


FIG.  15. — Single-phase  watthour 
demand  meter  (Westinghouse  Type 
RO). 


FIG.  16. — Polyphase  watthour 
demand  meter  (Westinghouse  Type 
RO). 


cerned,  simply  to  regulate  the  rate  of  deflection  of  the  auxiliary  disc  A 
and  the  progression  of  the  demand  pointer.  The  main  disc  supplies  no 
power  to  the  maximum-demand  mechanism  except  the  negligible  amount 
required  to  oscillate  the  escapement  claw. 

46.  The  Principle  of  Operation  of  the  RO  Demand  Meter  is 
this:  When  energy  flows  through  the  meter,  the  main  watt- 
hour-meter  disc  (B,  Fig.  17)  is  forced  to  rotate  the  same  as  in 
any  alternating-current  watt-hour  meter.  Its  rotational  speed 
is,  obviously,  proportional  to  the  load.  The  auxiliary  disc 
A,  which  drives  the  maximum-demand  pointer,  will  also  tend 
to  rotate.  However,  its  unrestricted  rotation  is  prevented  by 


30 


CENTRAL  STATIONS 


[AKT.  46 


the  action  of  the  escapement  wheel  F.  Now,  the  main  disc, 
driving  through  gears  JH  and  /  (which  have  no  connection 
with  the  auxiliary  shaft)  oscillates  the  escapement  by  means  of 
the  cam  G.  Thus,  the  auxiliary  disc  is  permitted  to  swing, 
but  at  a  speed  proportional  to  the  load.  As  the  escapement 
claw  oscillates,  the  teeth  of  the  escapement  wheel  are  allowed 
to  pass,  one  by  one,  until  the  tension  of  the  spiral  spring  C 
balances  the  rotating  torque  of  the  auxiliary  disc.  The  me- 
chanism is  then  in  equilibrium,  the  demand  pointer  indicating 
the  load.  But  although  the  main  disc  may  continue  to  rotate 


Timing  Gears,--      i 

H  Kimillillimdll  mUitM  /V^  \1 


n 


-LightSpring 


Escapement 

"Eccentric  Claw  \ 

Escapement  Wheel-' 
^•Auxiliary  Shaft 


Auxiliary  0/sc-v 


FIG.  17. — Diagrammatic  representation  of  mechanism  pf  Westinghouse  Type 
RO  demand  meter. 

so  long  as  the  load  is  maintained,  no  further  deflection  of  it 
(or  of  the  demand-indication  pointer  which  it  drives)  occurs, 
since  the  escapement  claw  is  now  oscillating  freely  between 
the  teeth  of  the  escapement  wheel. 

NOTE  that  the  escapement  wheel  F  is  not  driven  directly  from  the 
auxiliary  shaft  by  gear  K.  It  is  driven  by  a  ratchet  wheel  L  mounted 
on  a  sleeve,  which  is  loose,  on  the  auxiliary  shaft  and  to  which  the  gear 
K  is  attached.  This  ratchet  wheel  is  propelled  by  a  pawl  carried  by  an 
arm  fixed  to  the  auxiliary  shaft.  This  device  causes  the  deflection  of  the 
auxiliary  disc  to  be  retarded  by  the  escapement  wheel  when  the  pointer 
is  advancing  across  the  scale,  but  allows  the  disc  to  drop  back  freely  to 
equilibrium  when  the  load  is  reduced.  Hence,  the  auxiliary  disc  will 


SEC.  3]     MAXIMUM  DEMAND  AND  DEMAND  FACTORS        31 


•Hole  for  Supporting 


always  tend  to  follow  the  variation  in  load;  at  the  same  time  the  demand 
pointer  will  always  indicate  the  maximum  demand  up  to  the  instant  at 
which  the  instrument  was  observed. 

To  reset  the  instrument,  the  reset- 
ting button  T  is  pressed.  This  raises 
the  pawl  from  the  ratchet  and  the 
pointer  is  then,  by  the  action  of  the 
spiral  spring  S,  returned  to  zero  or  to 
the  position  of  the  auxiliary  disc. 

47.  The  Time-element  Feat- 
ure of  the  RO  Demand  Meter 
may  be  explained  thus:  When 
any    constant    load    is     being 
metered  by  the  instrument,  the 
time  required  for  the  auxiliary 
disc  to  reach  equilibrium  is  con- 
stant.    Thus,  assume  that  the 
demand  meter  is  so   calibrated 
and  adjusted  that  it  requires  just 
15  min.  to  reach  equilibrium  when 
the  constant  power  load  being 
metered  is  500  watts.     Then,  if 
this  500-watt  load  is  discontinued 
and  the  meter  cut  into  a  circuit 
carrying  a  1,000-watt  load  it  is 
obvious  that  the  demand  pointer 
will  deflect  over  exactly  twice  the 
angle  obtaining  with  a  500-watt 
load,  however,  the  main  disc  is 
now  rotating  at  twice  its  former 
speed.     Hence,  the  pointer  will 
attain  equilibrium  in  the  same 
time  interval  (15  min.)  as  with 
the  500-watt  load. 

48.  The  Wright  Demand 
Meter  (Figs.  18  and  19)  might 

be  classed  as  a  thermal-type  lagged  instrument.  It  indicates 
ampere  only  and  not  kilowatt  demands.  The  principle  is 
diagrammed  in  Fig.  19.  The  current,  or  the  shunted  part 


FIG. 


13. — The    Wright    demand 
meter. 


32 


CENTRAL  STATIONS 


[ART.  49 


thereof,  to  be  metered,  flows  through  the  resistor  coil  of  plati- 
noid, C,  which  is  wound  around  an  enlargement  A  of  a  U-- 
shaped hermetically  sealed  glass  tube.  The  tube  is  partially 
filled  with  sulphuric  acid  or  similar  fluid.  When  current  flows 
through  C,  the  resistor  is  heated  thereby.  The  air  within  the 
bulb  A  is  expanded  which  forces  the  liquid  to  rise  in  the  leg 
L.  If  the  liquid  rises  above  a  certain  height,  H,  it  flows  over 

into  the  small  graduated  index 
tube.  The  quantity  of  liquid 
which  flows  into  the  index  tube 
may  be  taken  to  indicate  the 
maximum  demand.  If  the 
index  tube  has  uniform  bore 
the  height  of  the  liquid  in  it 
will  then  be  a  measure  of  the 
square  of  the  maximum  current. 
In  the  actual  instrument  the 
divisions  increase  in  spacing 
from  the  bottom  up.  After 
having  been  read,  the  meter 
can  be  reset  by  tilting  the  tube, 
allowing  the  liquid  to  flow  back 
into  B.  These  meters  may  be 
used  on  either  alternating  or 
direct-current  circuits  and 
while  they  have  some  disadvan- 
tages there  are  a  great  many  of 
them  in  use.  The  design  of  the 
FIG.  19.— illustrating  the  principle  device  has  not  been  altered 
operation  of  the  Wright  demand  materially  in  years.  Since  the 

air  in  the  tube  heats  gradually 

the  instrument  provides  a  certain  time  lag.  Its  manufacturers 
state  that  if  a  steady  over-demand  continues  for  5  min.,  80 
per  cent,  of  the  load  will  be  indicated;  10  min.,  95  per  cent.; 
30  min.,  100  per  cent. 

49.  Thermal  or  Thermostatic  Meters  of  several  different 
types  have  been  proposed.  In  these  the  movement  of  the 
demand-meter  pointer  is  usually  actuated  by  the  expansion 


II 


SEC.  3]     MAXIMUM  DEMAND  AND  DEMAND  FACTORS       33 


of  the  fluid  or  gas.  They  are  all  classed  as  lagged-type  instru- 
ments. Figs.  9  and  10  show  typical  characteristic  curves  for 
instruments  of  this  general  character,  which  were  designed  or 
calibrated  for  15-min.  and  30-min.  time  intervals,  respectively. 
60.  Of  the  Integrating  Indicating  Demand  Meters  the 
General  Electric  Company's  M-4  for  alternating-current  and 
M-5  for  direct-current  service  (Figs.  20,  21  and  22)  are  typical. 
These  demand  meters  operate  in  conjunction  with  a  watt- 
hour  meter,  substantially  as  suggested  in  Fig.  22.  The 
demand-indicating  element  is  driven  electrically  from  the 
register  of  the  watt-hour  meter. 
Fig.  21  diagramming  the  mech- 
anism of  an  alternating-cur- 
rent meter,  illustrates  the  gen- 
eral principle. 

OPERATION. — A  contact  C  (Fig. 
21)  is  mounted  on  the  watt-hour 
meter.  It  completes  contact  each 
time  the  watt-hour  meter  has  made 
a  certain  number  of  revolutions, 
that  is,  every  time  a  certain  number 
of  kilowatt-hours  of  energy  has  been 
registered  by  the  watt-hour  meter. 
Whenever  C  contacts,  operating  coil 
0  is  energized  and  pulls  over  its 
armature.  This  advances  ratchet  R 
one  notch.  In  this  way  the  friction 
pointer  is  propelled  forward — the 
motion  being  transmitted  by  the 
ratio  gears — as  energy  is  consumed 
in  the  circuit  being  metered.  The  friction  pointer  is  not  rigidly  con- 
nected to  shaft  S,  but  is  pushed  by  the  dog  D,  which  is  rigidly  connected 
to  S.  It  follows  that  the  position  of  the  dial-pointer  on  the  scale  is  de- 
termined by  the  energy  consumption  registered  by  the  watt-hour  meter. 

However,  so  that  the  readings  of  the  demand  meter  will — to  satisfy 
the  definition  of  maximum  demand  quoted  above — indicate  the  demand 
over  a  definite  time  interval  it  is  imperative  that  the  dog  D  which  drives 
the  friction  pointer  over  the  scale  shall  be  reset  to  the  zero  position  at 
the  end  of  the  specified  time  interval.  But  the  dial  pointer  must  be  per- 
mitted to  remain  at  the  most  advanced  location  on  the  dial  scale  to  which 
it  has  been  forced  by  the  dog.  The  mechanism  of  this  timing  feature  is 
this: 


FIG.  20. — Alternating-current  de- 
mand meter  diagrammatic  repre- 
sentation of  which  is  shown  in  Fig. 
21  (General  Electric  Co.,  Type 
M-4). 


34 


CENTRAL  STATIONS 


[ART.  50 


The  aluminum  disc  M  (Fig.  21)  in  the  demand  meter  (which  is  rotated 
by  the  field  produced  by  an  alternating-current  electromagnet)  is  similar 
to  the  disc  in  any  alternating-current  watt-hour  meter  and  is  caused  to 
rotate  by  the  same  magnetic  reactions  as  those  which  rotate  the  disc  in 


FIG.  21. — Diagrammatic 
alternating-current    demand 
Form  AA).     The  corresponc 
except  that  the  disc  and  shaft 


representation  of  operating  mechanism  of 
meter.  (General  Electric  Co.,  Type  M-4, 
ng  direct-current  meter  (Form  BA)  is  similar 
s  replaced  by  a  clock  mechanism. 


such  a  watt-hour  meter.  Now,  since  the  torque  against  which  the  disc 
is  working  is  constant  and  since  the  frequency  of  the  alternating  current 
is  constant,  the  disc  always  rotates  at  a  constant  speed. 

The  mechanism  is  so  designed  and  calibrated  that  at  the  end  of  each 
of  the  time  intervals  for  which  the  meter  hasbeen  a  d justed  (for  example 


SEC.  3]     MAXIMUM  DEMAND  AND  DEMAND  FACTORS       35 


at  the  end  of  each  15-min.  interval  or  the  end  of  each  30-min.  interval) 
the  trip  levers  T,  by  the  action  of  the  cams  K,  will  be  pulled  together  by 


FIG.  22. — Method  of  connecting  alternating-current  demand  meter  for 
440  and  660-volt  loads.  For  110  and  220-volt  loads,  and  for  direct-current 
loads  the  transformer  is  omitted. 


The  cams  K  are  caused  to 


the  small  spring  in  tension  between  them, 
rotate  at  a  uniform  speed  by  disc  M 
which  drives  through  the  gearing 
shown.  When  the  trip  levers  are 
pulled  together  the  clutch  at  H  is 
released  and  the  spring  S  will  return 
the  dog  D  to  the  zero  position.  But 
the  friction  pointer  will  remain  where 
it  was.  Hence,  the  dog  is  thus  re- 
turned to  zero  at  the  end  of  each  of 
the  prescribed  time  intervals.  It  is 
obvious,  then,  that  the  direction 
pointer  will — assuming  that  its  scale 
is  properly  calibrated — always  in- 
dicate the  maximum  demand  for  the 
period  during  which  the  meter  has 
been  in  operation. 

After  the  maximum-demand  read- 
ing for  the  period— for  example,  for 
a  month — under  consideration  has 
been  taken,  the  demand  pointer  is 
returned  to  the  zero  position  by  the 
meter  reader.  He  unseals  the  re- 
setting device  and  turns  the  handle. 

The  direct-current  demand  meter  (M-5)   of  this  general 
type  is  similar  to  the  alternating-current  except  that  instead 


FIG.  23. — Printometer-type  de- 
mand meter  (General  Electric, 
Type  P,  Form  AA). 


36 


CENTRAL  STATIONS 


[ART.  51 


of  the  timing  mechanism  being  controlled  by  an  aluminum 
disc,  which  is  rotated  by  an  alternating-current  magnetic 
field,  it  is  rotated  by  a  clockwork  within  the  demand  meter. 
61.  Printometer-type  Demand  Meters  are  made  for  both 
alternating  and  direct-current  service.  The  printometer 
(Fig.  23  and  P,  Fig.  24)  is  not  of  itself  a  meter,  but  is  merely 


Watthour  Meter, 


, '"Demand  Meter 
— i 


^Contact-Making  Clock 
for  Making  Contact  at 
Predetermined  Time 
Intervals 


FIG.  24. — Diagrammatic  representation  of  operating  mechanism  of  printom- 
eter-type  demand  meter. 

a  registering  device  used  in  combination  with  a  standard 
watt-hour  meter  (W,  Fig.  24)  and  a  contact-making  clock 
(C,  Fig.  24  and  Fig.  25) .  These  illustrations  show  the  General 
Electric  Company's  Type  P-2  instrument.  Printometer 
records  are  printed  on  paper  strips,  a  part  of  one  of  which  is 
shown  in  Fig.  26.  This  record  shows  the  total  energy  con- 
sumption as  registered  by  the  watt-hour  meter.  It  also  shows 


SEC.  3]     MAXIMUM  DEMAND  AND  DEMAND  FACTORS       37 


v -White  '. 
•  .."?bfrs  and  Hands 
Black  D,.< 


During  each  30- 
Minute Interval 
«-  0 


the  time  of  day  at  which  the  various  blocks  of  energy  between 
any  two  successive  printings  were  consumed.     The  maximum 
demand  can  be  readily  determined  from  the  tape  record  (Fig. 
26)  which  also  will  indicate  the  hour 
and  date  upon  which  it  occurred. 

OPERATING  PRINCIPLE. — As  energy  flows 
through  the  watt-hour  meter  W  (Fig.  24) 
the  meter  contact  is  successively  closed  and 
opened  and,  whenever  it  is  closed,  current 
flows  through  the  solenoid  in  P  and  its 
plunger  forces  the  cyclometer  type  wheels 
ahead  a  notch.  The  mechanism  is  so  ar- 
ranged that  the  type  wheels  are  moved  for- 
ward at  a  rate  equal  to  the  rate  of  flow  of 
energy  through  the  watt-hour  meter.  The 
upper-most  figures  on  the  type  wheels  give 
at  any  instant  a  reading  in  kilowatt-hours 
equivalent  to  that  indicated  by  the  watt- 
hour  meter  dial. 


FIQ.  25.  —  Contact-making  clock  or 
contactor  for  printometer-type  demand 
meter. 


FIG.  26.  —  A  twelve-hour 
record  from  a  "  printometer  " 
demand  meter  (General  Elec- 
tric Co.  ,  Type  P)  .  Illustration 
is  exactly  half  actual  size. 


The  contactor  C  (Fig.  24)  on  the  clock  makes  contact  at  the  prede- 
termined time  intervals  (for  example,  every  15  min.  or  every  30  min.) 
and  each  time  that  it  does  the  kilowatt-hour  consumption  up  to  that 
instant  is  recorded  on  the  tape  as  shown  in  Fig.  26.  Simultaneously  the 

50279 


CENTRAL  STATIONS 


[ART.  52 


time  of  day  at  that  instant  is  printed  on  the  tape.  Thus  the  record  shown 
in  Fig.  26  is  obtained.  The  watt-hour-meter  contact  shown  at  W  (Fig. 
24)  can  be  attached  to  any  watt-hour  meter.  Where  the  printometer  is 
to  be  used  in  making  tests  at  a  number  of  different  locations  a  portable 
outfit  similar  to  that  of  Fig.  27  is  convenient. 

52.  An  Integrating  Graphic  Demand  Meter  (General 
Electric  Company's  Type  G-2)  is  shown  in  Figs.  28  and  29. 
The  chart  or  record  produced  by  this  device  is  reproduced  in 
Fig.  30  and  the  method  of  connecting  it  in  Fig.  31.  The 


•Top  Lug 


Stylu 


FIG.  27. — Printometer- type  de- 
mand meter  arranged  in  self-con- 
tained case  for  field  tests. 


FIG.  28. — A  graphic  demand  meter 
(General  Electric,  Type  G-2). 


demand-meter  movement  is  controlled  by  a  watt-hour  meter 
with  which  it  is  electrically  interconnected  (Fig.  29).  The 
contactor,  which  is  mounted  on  the  watt-hour-meter  register 
W,  closes  the  control  circuit  at  intervals,  the  frequency  of 
which  is  determined  by  the  speed  of  rotation  of  the  watt- 
hour-meter  disc,  that  is  by  the  rate  of  energy  consumption 
in  the  metered  circuit. 

OPERATION. — Each  time  the  contactor  closes  the  control  circuit,  the 
operating  coil  in  the  demand  meter  G  (Fig.  29)  is  energized  and  its  arma- 


SEC.  3]     MAXIMUM  DEMAND  AND  DEMAND  FACTORS       39 


FIG.  29. — Schematic  diagram  of  mechanism  of  graphic  demand  meter. 


FIG.  30. — Graphic   record   produced   by   the   General   Electric   Company's 
Type  G  demand  meter  of  Fig.  23. 


40 


CENTRAL  STATIONS 


[ART.  52 


ture  lever  is  attracted.  This  moves  the  ratchet  wheel  forward  a  notch. 
When  the  contactor  opens,  the  coil  is  deenergized  and  the  lever  is  pulled 
back  by  its  spring — ready  for  another  push.  The  stylus  is  moved  upward 
a  certain  distance  each  time  the  contactor  closes,  the  motion  being  trans- 
mitted to  the  stylus  through  the  gear  train  shown. 

As  energy  is  consumed  in  the  metered  circuit  and  the  contactor  opens 
and  closes,  the  stylus  is  forced  upward  "step  by  step"  across  the  chart 
until  the  end  of  the  time  interval  for  which  the  meter  has  been  adjusted. 
At  the  end  of  each  time  interval,  the  cam  (which  is  driven  by  the  clock 
within  the  meter  which  also  rotates  the  chart)  has  rotated  to  such  a  posi- 
tion that  the  first  of  the  trip  levers  falls.  This  disengages  the  sliding 
pinion  from  the  gear  in  which  it  normally  meshes,  which  permits  the  set- 
back spring  to  pull  the  stylus  mechanism  to  the  zero  position. 

As  the  rotation  of  the  cam  is  continued,  the  second  trip  lever  is  per- 
mitted to  drop  which  returns  the  sliding  pinion  to  its  normal  position  and 


FIG.  31. — Method  of  connecting  alternating-current  graphic  demand 
meter  for  440-volt  circuits.  For  220  and  110-volt  alternating-current  loads 
and  for  direct-current  loads,  the  transformer  is  omitted. 


again  completes  the  gear  transmission  system  between  the  armature 
lever  and  the  stylus.  This  having  been  effected  the  demand-meter 
movement  is  in  trim  to  again  drive  the  stylus  over  the  chart  during  the 
next  time  interval. 

Since  the  clock  within  the  demand  meter  rotates  the  chart  at  constant 
speed,  the  stylus  will  travel  a  different  course  over  the  chart  during  each 
time  interval  as  shown  in  Fig.  30.  In  any  time  interval  the  distance 
(measured  along  the  curved  line  which  it  draws)  of  the  stylus  from  the 
zero  circle  of  the  chart  is,  at  any  instant,  directly  proportional  to  the 
number  of  kilowatt-hours  energy  registered  by  the  controlling  watt-hour 
meter  during  that  interval.  Hence,  the  ends  of  stylus-record  lines  indi- 
cate the  demands  for  the  different  intervals.  The  end  of  the  longest 
stylus-record  line  indicates  the  maximum  demand.  Thus,  the  maximum 
demand  for  the  period  comprehended  by  the  graph  of  Fig.  30  is  (at  M) 
98  kw. 


SEC.  3]     MAXIMUM  DEMAND  AND  DEMAND  FACTORS        41 


52a.  The    Westinghouse    Recording-demand    Watt-hour 
Meter  is  shown  in  Figs.  32  and  33.     This  instrument  combines 


Wat 'hour 
Meter  Dial, 


6  lass  Window 


'Record  Paper 


FIG.  32. — The   Westinghouse   Type          FIG.  33. — The  mechanism  of  the 
RA  demand  watthour  meter.  Westinghouse    Type    RA     demand 

watthour  meter. 

in  one  unit  a  watt-hour  meter  and  a  demand  meter.     It 
indicates,*  on  a  four-dial   counter,  the  total  kilowatt-hours 

,-Houn  of  Day  Stamped  onPuper 


'•Perforations  to  Engage 
Pins  in  Driving  Drum 

FIG.  34. — Record  of  a  15-minute-     FIG.  35. — Record    of    a     30-minute- 
interval  RA  meter.  interval  RA  meter. 

energy  consumption  and  records,  on  a  strip  or  ribbon  of  paper 
(Figs.  34  and  35),  the  integrated  kilowatts  demand  over  suc- 
cessive predetermined  time  intervals. 

'  Westinghousc  Electric  &  Manufacturing  Company. 


42  CENTRAL  STATIONS  [ART.  53 

PRINCIPLE  OP  OPERATION. — Under  load,  the  gear  train  of  the  watt- 
hour  meter  advances  the  counters  in  the  regular  manner.  At  the  same 
time  the  gear  train  causes  the  ink-carrying  pen  to  advance  across  the 
record  paper  in  proportion  to  the  energy  registered.  At  the  end  of  a 
predetermined  time  interval  a  stud  on  the  reset  wheel  releases  the  pen 
gear  from  mesh  with  the  gear  train  and  a  balancing  weight  returns  the 
pen  to  zero,  where  it  is  again  meshed  with  the  gear  train  to  repeat  its 
advance  during  the  next  time  interval. 

Just  before  the  pen  gear  is  released,  the  record  paper  is  advanced 
Ke  in-  by  the  operating  spring  so  that  the  pen  makes  a  distinct  and 
readily  observed  record  of  the  maximum  pen  travel.  This  record  shows 
both  the  amount  of  integrated  demand  and  (by  the  time  calibration 
printed  on  the  record  paper)  the  time  of  its  occurrence.  The  meter 
may  be  geared  for  15,  30  and  60-min.  time  intervals. 

53.  Demand  Factor*  is  "the  ratio  of  the  maximum  demand 
of  any  system  or  part  of  a  system,  to  the  total  connected  load 
of  the  system  or  that  part  of  the  system  under  consideration." 
It  is  expressed  as  a  percentage.     Thus,  to  obtain  the  "  demand 
factor"    of   any  installation,   divide   the  maximum  demand 
(usually  expressed  in  watts  or  in  kilowatts),  imposed  by  the 
connected  load,  by  the  connected  load  (correspondingly  ex- 
pressed in  watts  or  in  kilowatts).     Thus: 

,-,  ~  ,  ,    .          maximum  demand 

(7)  Demand  factor  =  — 

connected  load. 

(8)  Maximum  demand  =  (demand factor)  X  (connected load). 

/r.v  -,  .   ,  7      ,       maximum  demand 

(9)  Connected  load  =  — 

demand  factor. 

54.  The  Explanation  of  "Demand  Factor"  will  now  be  con- 
sidered: It  is  seldom  that  the  kilowatt  or  kilovolt-ampere 
maximum  power  demand  of  a  group  of  electrical  devices  or 
"receivers"  is  equal  to  the  sum  of  the  kilowatt  or  kilovolt- 
ampere  ratings  or  capacities  of  the  receivers.     There  are  two 
reasons  for  this  condition.    First:  Electrical  apparatus  is  fre- 
quently selected  of  a  size  greater  than  is  actually  necessary  to 
perform  the  duties  imposed  on  it.     That  is,  excess  overload 
or  reserve  capacity  is  provided.     Second:  In  a  group  of  elec- 
trical devices,  it  does  not  often  occur  that  all  of  the  devices  will, 

•  A.  I.  E.  E.  STANDARDIZATION  RULES. 


SEC.  3]     MAXIMUM  DEMAND  AND  DEMAND  FACTORS        43 

at  the  same  time,  be  imposing  the  maximum  loads  which  each 
can  impose  on  the  supply  circuit. 

NOTE. — It  follows  from  the  definition  of  demand  factor  recited  above, 
that  demand  factors  are  most  often  less  than  100  per  cent.  Sometimes, 
however  (see  accompanying  tables),  100  per  cent,  demand  factors  are 
encountered.  Also,  demand  factors  may  be,  and  sometimes  are,  greater 
than  100  per  cent.  Where  such  a  condition  exists,  some  of  the  receiving 
apparatus  must,  obviously,  be  overloaded. 

55.  The  Determination  of  a  Demand  Factor  can  be  readily 
effected  if  the  two  values:  (1)  The  Maximum  Demand  and  (2) 
The  Connected  Load,  are  known.     Hence,  in  finding  the  de- 
mand factor  of  an  existing  installation,  its  maximum  demand 
must  be  measured — ascertained  by  test — by  following  one  of 
the   methods   hereinbefore   described.     Then   the  connected 
load  can  be  computed  by  adding  together  the  nameplates  or 
manufacturers'  ratings  of  all  of  the  receiver  devices  in  the  in- 
stallation.    The  values  thus  obtained  may  then  be  substituted 
in  equation  (4)  and  the  problem  solved.     Some  of  the  examples 
under  Art.  58  illustrate  the  process.     In  designing  new  in- 
stallations where  it  is  necessary  to  know  the  maximum  de- 
mands, the  designer  must  assume  demand  factors,  basing  his 
assumptions  on  values  which  tests  have  shown  to  obtain  under 
similar  conditions  in  practice.     The  values  given  in  the  fol- 
lowing tables  may  be  used  as  a  basis  for  such  assumptions. 

56.  In  Determining  Demand  Factors  of  Direct-current  Cir- 
cuits it  is,  where  feasible,  desirable  to  reckon  the  connected 
load  and  the  maximum  demand  in  watts  or  in  kilowatts. 
However,  where  the  maximum  demand  can  be  more  conveni- 
ently ascertained   in   amperes,  then   the  connected  load  is 
taken  as  the  sum  of  the  full-load  current  (in  amperes)  inputs  of 
all  of  the  devices  in  the  group  under  consideration.     In  such 
cases  it  is  assumed  that  the  voltage  impressed  on  the  multiple 
"constant-potential"  circuit  remains  constant.     Where  the 
maximum-demand  values  have  been  observed  by  test  with  a 
direct-current,  line-testing  ammeter,  like  that  of  Fig.  12,  the 
connected  load  is  most  conveniently  taken  in  amperes. 

57.  In  Determining  Demand  Factors  of  Alternating-current 
Circuits  the  maximum  demand  may  be  observed  in  either 


44  CENTRAL  STATIONS  [ART.  58 

kilowatts,  kilovolt  amperes,  or  amperes — depending  upon  the 
methods  and  instruments  used  for  measuring  the  maximum 
demand.  All  of  the  maximum-demand  meters  now  on  the 
market,  except  those  of  the  thermal  types,  indicate  the  maxi- 
mum demand  in  watts  or  in  kilowatts.  Where  ammeters, 
similar  to  those  of  Figs.  13  or  14  are  used,  the  maximum- 
demand  value  thereby  obtained  is,  obviously,  in  amperes. 
The  thermal-type  demand  meters  usually  read  in  amperes. 
Where  the  maximum  demand  is  measured  in  amperes,  it  is 
multiplied  by  the  impressed  voltage  to  obtain  the  kilovolt- 
ampere  equivalent. 

NOTE. — The  connected  load  may  be  reckoned  in  either  kilowatts  or 
kilovolt-amperes.  It  is  however,  usually  reckoned  in  kilowatts  because 
most  receiving  devices  are  rated  in  kilowatts — or  in  watts  or  horse-power, 
both  of  which  may  be  directly  reduced  to  kilowatts. 

Hence,  it  follows  that  the  demand  factor  of  an  alternating-current 
circuit  may  be  taken  as  either:  (1)  (kw.  max.  dem.)  -f-  (kw.  con.  load], 
or  (2)  as  (kva.  max.  dem.)  -f-  (kw.  con.  load).  It  is,  in  general,  the  kilovolt- 
ampere  demand  rather  than  the  kilowatt  demand  which  determines  the 
capacity  of — hence,  the  investment  required  for — the  electrical  equip- 
ment required  to  serve  a  given  load.  Hence,  the  second  method  (kva. 
max.  dem.)  -5-  (kw.  con.  load)  appears  to  be  the  more  logical  one.  It  is 
the  one  which  must,  directly  or  indirectly,  be  used  where  the  maximum- 
demand  determination  is  made  to  provide  a  basis  for  the  selection  of 
equipment  and  plant  to  serve  a  load.  However,  since  most  demand 
indicators  indicate  kilowatts  instead  of  kilovolt-ampere  electricity  rate 
schedules  are,  probably,  most  often  based  on  a  "  (kw.  max.  dem.)  -5-  (kw. 
con.  load) "  demand  factor.  In  such  cases  the  feature  of  the  power  factor 
of  the  load  is  recognized  in  some  arbitrary  way  in  the  rate  schedule  which 
is  compiled  to  apply. 

58.  How  Demand  Factors  are  Used  is  illustrated  in  the 
examples  which  follow.  Broadly,  demand  factors  have  only 
one  general  application,  that  is,  to  determine  the  capacity — 
hence  cost — of  the  apparatus  which  will  be  required  to  serve 
a  given  load.  As  above  suggested,  they  are  also  used  in  com- 
puting rate  schedules  but  they  should  be  factors  in  such 
computations  only  because  of  their  influence  on  the  required 
investment. 

EXAMPLE. — A  certain  residence  has  a  connected  load  as  follows: 
four  60-watt  lamps,  twenty  40-watt  lamps,  six  10-watt  lamps.  With  a 


SEC  3]     MAXIMUM  DEMAND  AND  DEMAND  FACTORS        4i> 


46  CENTRAL  STATIONS  [AST.  59 

demand  meter  it  is  observed  that  the  30-min.  maximum  demand  is  838 
watts.  What  is  the  demand  factor  of  this  installation?  SOLUTION. — 
Connected  load  is:  (4  X  60)  +  (20  X  40)  +  (6  X  10)  =  240  +  800  +  60 
=  1,100  watts.  Then  substituting  in  equation  (7)  Demand  factor  = 
(maximum  demand)  -f-  (connected  load)  =  638  H-  1,100  =  0.58.  Hence, 
the  demand  factor  of  this  installation  on  the  basis  of  a  30-min  .-interval 
maximum  demand  is  58  per  cent. 

EXAMPLE. — The  connected  lighting  load  in  a  large  theatre  is  3.6  kw. 
What  will,  probably,  be  the  30-min.  maximum  demand?  SOLUTION. — 
The  probable  demand  factor  for  a  lar-ge  theatre  lighting  load  is  (Table 
61)  60  per  cent.  Hence,  substituting  in  the  equation  (8):  Maximum 
demand  =  (demand  factor)  X  (connected  load)  =  0.60  X  3,600  =  2,160 
watts.  Hence,  the  30-min.  maximum  demand  would  be,  probably,  about 
2.2  kw. 

EXAMPLE. — In  Fig.  36  is  shown  a  group  of  motors  representing  a 
total  connected  load  of  370  h.p.  Assuming  that  for  installations  of  this 
character  the  demand  factor  is  known  to  be  55  per  cent.,  what  is  the 
maximum  demand?  SOLUTION. — In  equation  (8):  Maximum  demand 
=  (demand  factor)  X  (connected  load)  =  0.55  X  370  =  203.5  h.p. 
Now,  203.5  X  0.746  =  152  kw.,  which  is  the  maximum  demand. 

EXAMPLE. — In  Fig.  37  the  total  connected  load,  that  is,  the  sum  of  the 
watts  load  installed  in  all  of  the  buildings,  is  16.75  kw.  The  maximum 
demand  indicated  by  the  wattmeter  P,  that  is,  the  greatest  demand  ever 
indicated  by  this  instrument  is  6.7  kw.  What  is  the  demand  factor  for 
this  load?  SOLUTION. — Substitute  in  (7):  Demand  factor  =  (maximum 
demand)  +  (connected  load)  =  6.7  -^  16.75  =  0.40  =  40  per  cent. 
Hence,  the  demand  factor  for  this  group  of  consumers  is  40  per  cent. 

EXAMPLE. — In  a  certain  town  of  1,000  inhabitants  in  Missouri  served 
by  an  alternating-current  plant  the  connected  load,  motors,  lamps  and 
all  receiving  devices  totals  55  kw.  The  30-min.  maximum  demand  is 
20.8  kva.  What  is  the  demand  factor  for  the  system?  SOLUTION. — 
Substitute  in  equation  (7):  Demand  factor  —  (maximum  demand)  -=- 
(connected  load)  =  20.8  -f-  55  =  0.38.  Hence,  the  30-min.  interval 
demand  factor  for  this  installation  is  38  per  cent. 

59.  The  Tables  of  Demand  Factors  which  are  given  here- 
with are  intended  merely  to  serve  as  guides.  While  it  is 
believed  that  they  represent  average  conditions  they  must  be 
used  with  judgment.  The  only  certain  way  to  determine 
demand  factors  for  specified  conditions  is  to  ascertain  them 
by  test.  It  is  as  impracticable  to  arrange  a  table  of  demand 
factors  which  will  cover  all  conditions  as  it  would  be  to  com- 
pile a  schedule  indicating  what  kind  of  a  hat  a  woman  of  a 
certain  complexion  and  nationality  would  probably  buy  under 


SEC.  3]     MAXIMUM  DEMAND  AND  DEMAND  FACTORS       47 

given  conditions.  However,  where  test  values  are  unknown 
the  factors  tabulated  (Tables  61  to  65)  should  be  of  service 
in  designing  new  installations  and  plants. 

60.  Demand  Factors  for  Lighting  Installations  are  reason- 
ably constant  for  each  of  the  different  classes  of  service.     For 
example,  the  demand  factor  for  saloons  will  usually  be  in  the 
neighborhood  of  that  indicated  in  Table  61;  that  is,  about  70 
per  cent.     Furthermore,  these  lighting-installation  demand 
factors  would  be  about  the  same  for  a  given  installation  from 
week  to  week.    Lighting  loads  are  not  subject  to  the  sudden 
pronounced    variations    that    occur    with    power    demands. 
Hence,  the  demand  factor  for  a  lighting  load  determined  on 
a   30-min.-interval    basis    will    be    about    the   same  as  one 
determined  for  the  same  load  on  a  15-min.-interval  basis. 

61.  Approximate  Demand  Factors  for  Miscellaneous  Light- 
ing Service. — Factors  are  based  on  observed  data  from  a 
number  of  sources  on  a  30-min. -interval  maximum  demand. 
The  following  factors  apply  to  one  consumer  only.     That  is, 
to  obtain  the  "30-min."  maximum  demand  of  one  consumer 
of  any  of  the  given  classes,  multiply  his  connected  load  in 
watts   by   the    corresponding    demand   factor.     Factors  de- 
termined on  a  15-min. -interval  basis  would  not,  probably, 
differ  materially  from  those  given. 


Demand  fact 

ore  in  per  cent. 

Class  of  lighting  service 

Probable 
range 

Probable  fair 
average  value 

Sign,  outline,  display  and  window  lighting.  .  .  . 
Theatres  (small)  
Offices  (business  and  professional)  
Banks 

90-100 
70-90 
55-90 
55-85 

100 
75 
70 
70 

Saloons  

60-90 

70 

Restaurants  
Laundries  
Lodge  and  dance  halls  
Depots  (railway  stations) 

50-80 
60-75 
65-90 
75-95 

70 
70 
70 
70 

Shops  (barber  shops  and  the  like) 

55-80 

70 

48 


CENTRAL  STATIONS 


[ART.  62 


Demand  fac 

ors  in  per  cent. 

Class  of  lighting  service 

Probable 
range 

Probable  fair 
average  value 

Stores  . 

40-95 

65 

Pool  and  billiard  rooms  
Printers  and  engravers  shops  
Theatres  (large)  

40-70 
30-75 
40-75 

65 

60 
60 

Churches  and  auditoriums  
Factories  

55-85 
45-60 

60 
55 

Livery  stables  

50-60 

55 

Schools  used  at  night  

35-55 

50 

Machine  shops 

25-60 

45 

Hospitals 

25-60 

45 

Warehouses  .  . 

20-45 

40 

County,  federal  and  municipal  buildings  
Universities  and  colleges  

30-40 
20-50 

35 
30 

62.  Approximate  Demand  Factors  for  Small  Lighting  Con- 
sumers.— The  following  factors  are  those  said  to  be  used  in 
Chicago  for  computing  rates  for  small  lighting  customers. 
They  may  be  taken  as  fairly  representing  average  small- 
lighting-customer  conditions.  It  is  understood  that  these 
factors  are  based  on  a  large  number  of  observations  made  with 
Wright  demand  meters.  They  are,  therefore,  approximately, 
on  the  basis  of  a  15-min. -interval  maximum  demand.  How- 
ever, it  is  likely  that,  for  this  class  of  service  (lighting  service), 
the  demand  factors  would  not  be  materially  different  from 
those  tabulated  if  they  were  based  on  a  30-min. -interval 
maximum  demand. 


SEC.  3]     MAXIMUM  DEMAND  AND  DEMAND  FACTORS        49 


Connected  load, 
watts 

Demand  factors,  per  cent. 

A*  Chicago  values  used  for  com- 
puting rates  of  small  consumers 

B*   Values   of  graph   of   Fig.   38, 
averaged  from  Chicago  values 

Commercial 

Residence 

Commercial 

Residence 

250 

100 

100 

100 

100 

300                    100 

89 

100 

93 

350 
400 

95 

91 

86 

83 

94 
91 

86 
80 

450 

89 

74 

88 

85 

500 

87 

73 

86 

71 

550 

85 

67 

84 

67 

600 

83 

67 

83 

65 

650 

82 

61 

82 

62 

700 

81 

61 

81 

60 

750 

80 

57 

80 

58 

800 

79 

57 

79 

57 

850                     78 

55 

78 

56 

900                     78 

ncn                              T7 

55 

CO 

78 

TT 

54 

CO 

*  The  values  in  the  columns  headed  A  are  those  which  have  been  used 
n  Chicago.  These  values  were  plotted  in  Fig.  38.  Then  the  smooth 
jurves  were  drawn  through  them.  The  average  taken  from  these  smooth 
rraph  values  are  given  in  the  columns  headed  B. 


300        400          500         600 
Watts  Connected  Load 

FIG.  38. — Graph  showing  relation  of  demand  factor  of  small  consumers  to 
connected  load. 

63.  Demand  Factors  for  Motor  Installations  are  subject  to 
considerable  variation  and  hence,  should,  in  all  important 


50 


CENTRAL  STATIONS 


[ART.  64 


studies,  be  determined  by  test.  To  be  of  material  value 
such  test  should  extend  over  an  extended  period  because  ex- 
perience has  shown  that  the  demand  of  motor  loads  may  be 
very  much  greater  on  certain  days  or  months  than  on  others. 
64.  Approximate  Demand  Factors  for  Alternating-current 
Motor  Installations. — Factors  are  based  on  observed  data 
from  a  number  of  sources  on  a  30-min.-interval  maximum  de- 
mand. In  this  table  it  is  assumed  that  (equation  7):  De- 
mand factor  =  (maximum  demand  in  kva.}  -\-  (connected  load 
in  kw.}.  Because  of  the  large  starting  currents  taken  by 
alternating-current  motors  the  demand  factors  involved  are 
liable  to  be  rather  high. 


Demand   factors   in 

Total 

per  cent. 

Number 

motoi 

Class  of  service 

of  motors 
in  installa- 
tion 

horse- 
powei  of 
installa- 

Probable 

Probable 
fair 

tion 

range 

average 
value 

Single-phase  and  three-phase  al- 

ternating-current motors             !      1—10 

1-75 

80-110 

90 

General  factory  and  other  service 

10-20 

1-150 

75-95 

85 

and  over 

| 

Small  single-phase  motors  

1-20 

1-50 

80-100 

90 

Alternating-current  elevator     I 
and  crane  motors. 

1-2 
3-5 
over  5 

90-110 
60-80 
50-70 

100 
70 
60 

65.  Approximate  Demand  Factors  for  Direct-current  Motor 
Installations. — Factors  are  based  on  observed  data  from  a 
number  of  sources  on  a  short-time  interval  maximum  demand. 
In  this  table  (equation  7):  demand  factor  =  (maximum 
demand  in  kw.)  +  (connected  load  in  kw.}. 


SEC.  3]     MAXIMUM  DEMAND  AND  DEMAND  FACTORS        51 


Demand  factors  in  per  cent. 

Number    of 

Total  motor 

Class  of  service 

motors  in 
installation 

horse-power 
of  installation 

Probable 

Probable  fair 

range 

average  value 

1 

1-5 

75-95 

85 

1 

6-10 

65-85 

75 

1 

11-20 

55-75 

65 

1 

over  20 

50-70 

60 

2 

1-  5 

70-90 

80 

2 

6-10 

65-85 

75 

Direct-current                2 

11-20 

60-80 

70 

motors,                      2 

over  20 

45-65 

55 

general  factory 

and  other 

3-5 

1-  5 

60-80 

70 

service 

3-5 

6-10 

55-75 

65 

3-5 

11-20 

50-70 

60 

3-5 

over  20 

40-60 

50 

6  and  over 

1-  5 

55-75 

65 

6  and  over 

6-10 

50-70 

60 

6  and  over 

11-20 

45-65 

55 

6  and  over 

over  20 

25-55 

45 

Machine  shop 

10  and  over 

over  20 

35-60 

40 

individual   drive 

i 

Elevator  and 
crane  motors 

I   M 

3-5 
[  over  5 

90-110 
60-80 
50-70 

100 
70 
60 

66.  The  Importance  of  Maximum  Demand  and  Demand 
Factor  in  Determining  Suitable  Transformer  Capacities  is  a 

thing  that  is  not  ordinarily  given  the  consideration  which  it 
deserves.  The  usual  tendency  when  installing  transformers 
serving  individual  loads — and  those  for  serving  group  loads 
for  that  matter — is  to  select  transformers  of  capacities  con- 
siderably larger  than  is  actually  necessary.  Hence,  where  a 
transformer  is  to  be  installed  some  sort  of  a  study  should 
always  be  made  to  ascertain  the  facts  and  determine  the  con- 
sumers actual  maximum  demand.  It  will  be  found  that  in 


52  CENTRAL  STATIONS  [ART.  66 

the  long  run  such  a  course  will  be  justified  by  the  saving  in 
fixed  charges  and  operating  expenses  which  will  result. 

NOTE. — Where  transformers  are  larger  than  actually  necessary,  the 
interest  charge  is  greater  than  it  should  be  and  there  are  also  superfluous 
charges  due  to  excessive  electrical  losses  which,  in  the  aggregate,  may  be 
considerable.  Where  a  group  of  consumers  is  to  be  served  by  a  trans- 
former it  is  desirable  to  base  the  capacity  of  the  transformer  to  be  in- 
stalled on  the  diversity  factor  (Art.  70)  of  the  consumers  as  well  as  on 
their  demand  factors. 


SECTION  4 
DIVERSITY  AND  DIVERSITY  FACTORS 

67.  Diversity,  as  defined  in  the  dictionary,  "is  the  state  of 
being  dissimilar  to  one  another."  There  is  a  "diversity" 
or  difference  among:  (1)  the  characteristics  of  the  different  loads 
of  the  same  general  class;  and  (2)  the  various  general  classes  of 
loads  which  a  central  station  system  may  be  called  upon  to 


400 


P.M. 

Fio.  39. — Graph  showing  diversity  between  power  and  lighting  loads. 

serve.  And,  as  will  be  shown  later,  it  is,  from  an  economic 
aspect,  extremely  fortunate  that  such  is  the  case.  In  central- 
station  parlance,  the  term  "diversity"  is  used  to  signify 
diversity  of  demand — to  imply  that  the  maximum  demands  of 
the  various  consumers  of  the  different  classes  and  of  the  differ- 
ent circuit  elements  in  an  energy-distribution  system  are  not 
coincident.  That  is,  their  different  maximum  demands  occur 
at  different  times,  and  not  simultaneously. 
53 


54 


CENTRAL  STATIONS 


[ART.  67 


FOR  ILLUSTRATION. — It  is  ob- 
vious that  the  residence-lighting 
load  will  attain  its  maximum  in 
the  evening,  whereas  a  manu- 
facturing establishment  will  or- 
dinarily require  the  greatest 
power  during  the  daylight  hours. 
Again,  commercial  establish- 
ments of  certain  types,  for  ex- 
ample department  stores,  gen- 
erally use  much  more  power 
during  the  day  than  in  the  even- 
ing, while  stores  of  other  sorts, 
such  as  drug  stores,  use  more 
in  the  evening.  A  similar  condi- 
tion holds  for  the  different  sorts 
of  manufacturing  establishments. 
Hence,  there  is  a  diversity  of 
demand  among  these  different 
classes  of  central-station  load. 
Fig.  39  illustrates  the  general 
idea  graphically.  It  shows 
typical  power  and  lighting-load 
graphs  for  an  average  city  and 
also  indicates  the  total-load  graph 
which  is  obtained  by  adding  to- 
gether those  for  the  power  and 
the  lighting  loads.  It  will  be 
noted  that  the  power-load  peak 
and  the  lighting-load  peak  do 
not  occur  at  the  same  time. 
In  other  words,  the  maximum 
demand  of  the  power  load  oc- 
curs at  a  different  time  from 
that  of  the  lighting  load.  There 
is  a  diversity  between  their 
maximum  demands. 

It  follows  that  the  maxi- 
mum demand  on  a  trans- 
former is  less  than  the  sum 
of  the  maximum  demands 
of  the  consumers  served 
from  that  transformer. 


SEC.  4]          DIVERSITY  AND  DIVERSITY  FACTORS  55 

Also,  the  maximum  demand  imposed  on  a  feeder  is  less  than 
the  sum  of  the  maximum  demands  of  the  transformers  fed 
from  the  feeder  and  the  maximum  demand  on  the  generating 
station  is  less  than  the  sum  of  the  maximum  demands  of  the 
feeders  supplied  from  the  station. 

68.  A    Graphic   Illustration  of  Diversity  of  Demand  is 
given  in  Fig.  40  which  was  constructed  from  data  obtained 
from  a  study  of  diversity.*     For  the  purpose  of  this  study  82 
consumers  were  classed  into  11  groups.     The  maximum  de- 
mand of  each  of  the  11  groups  occurring  during  a  certain  year, 
was  ascertained  with  maximum  demand  meters.     The  total 
area  of  each  of  the  rectangles,  1  to  11,  in  the  diagram,  is  pro- 
portional to  the  maximum  demand  of  the  corresponding  group 
of  consumers.     Then  the  combined — or  simultaneous — maxi- 
mum demand  during  the  same  year  for  all  of  the  82  consumers 
was   found.     The   combined   maximum   demand   for   all   of 
the  82  consumers  was  9,770  kw.  and  it  occurred  about  5:00  P.M. 
in  December.     At  the  same  hour  at  which  this  9,770  kw.  was 
imposed  on  the  central  station,  the  maximum  demands  of  the 
different  groups  were  proportional  to  the  shaded  areas  in  the 
small  rectanges.     It  will  be  noted  that  consumers  of  certain 
classes — such  as  the  brick  yards,  quarries,  ice  manufacturers 
and  cement  works — imposed  practically  no  demand  on  the 
system  at  the  time  (5:00  P.M.  in  December)  when  the  aggre- 
gate demand  of  all  of  the  82  consumers  was  a  maximum — 
9,770  kw. 

69.  The  Importance  of  the  Concept  of  Diversity  of  Demand 
can  be  appreciated  if  one  considers  the  increase  in  generating 
and  distributing  — plant  capacity  that  would  be  necessary  if  the 
maximum  demands  of  all  consumers  occurred  simultaneously. 
This  matter  of  diversity  is,  therefore,  of  great  economic  signi- 
ficance.    It  is  also  of  concern  to  the  engineer — because  a  de- 
signer should  consider  it  in  planning  his  generating  and  sub- 
stations and  his  distribution  plant.     Diversity  is  an  element  in 
the  determination  of  rates  for  electric  service.     If  it  were  not 
for  the  fact  that  the  combined  maximum  demand  imposed  on 

•  Discussed   by  Samuel   Insull  in   a  paper  ''CENTRALIZATION  OF  ENERGY  SUPPLT," 
delivered  before  the  Finance  Forum  of  the  New  York  Y.  M.  C.  A.,  on  April  20.  1914. 


56  CENTRAL  STATIONS  [ART.  70 

an  average  central  station  is  usually  considerable  less  than  half 
of  the  sum  of  the  maximum  demands  of  all  of  the  consumers" 
the  investment  involved  to  provide  electric  service  would  be 
very  much  greater  than  that  which  is  now  required.  If  it 
were  necessary  to  thus  increase  the  investment,  the  cost  of 
service  would  have  to  be  increased  accordingly. 

70.  The  Diversity  Factor  of  a  System  may  be  denned*  as 
"the  ratio  of  the  sum  of  the  maximum  power  demands  of  the 
sub-division  of  any  system  or  parts  of  a  system  to  the  maxi- 
mum demand  of  the  whole  system  or  of  part  of  the  system 
under  consideration  measured  at  the  point  of  supply."  In 
other  words,  a  diversity  factor  is  the  ratio  of  the  sum  of  the 
individual  maximum  demands  of  a  number  of  loads  during  a 
specified  period  to  the  simultaneous  maximum  demand  of  all 
these  same  loads  during  the  same  period.  If  all  of  the  loads 
in  a  group  impose  their  maximum  demands  at  the  same  time 
then  the  diversity  factor  of  that  group  will  be  one  (1). 

EXAMPLE. — Consider  two  consumers  each  of  which  has  a  maximum 
demand  for  100  kw.  The  sum  of  their  individual  maximum  demands 
would  then  be  200  kw.,  but  if  a  maximum  demand  meter  in  the  circuit 
supplying  these  two  consumers  indicated  only  150  kw. — as  it  might  if  the 
individual  demands  of  the  two  consumers  did  not  occur  at  the  same  time 
— the  diversity  factor  between  these  two  consumers  would  then  be: 
200  kw.  -f-  150  kw.  =  1.33. 

It  follows  then  that: 

fin\   r\-       •*    f    *  sum  of  individual  max.  demands 

(10)  Diversity  factor  = ^- -, -=— -. — - —      — 

maximum  demand  of  entire  group 

(11)  Sum  of  ind.  max.  dem.  =  (diversity  factor)  X 

(max.  dem.  of  entire  group). 

/m\    if       j          f  sum  of  ind.  max.  dmds 

(12)  Max  dem.  of  entire  qroup  =  =-^ : : • 

diversity  factor 

NOTE. — A  diversity  factor  is  sometimes  given  as  the  reciprocal  of  the 
value  obtained  from  the  above  equations.  That  is,  in  such  cases  it  is 
taken  that:  Diversity  factor  =  (max.  dem.  of  entire  group)  -j-  (sum  of 
ind.  demands').  The  factor  thus  obtained  will,  in  every  case,  be  equal  to 
unity  (one)  or  less.  Such  a  factor  is,  usually,  more  convenient  of  applica- 
tion than  is  one  derived  by  using  equation  (10).  However,  equation 

•A.  I.  E.  E.  STANDARDIZATION  RULES,  Sec.  60. 


SEC.  4]          DIVERSITY  AND  DIVERSITY  FACTORS 


57 


(10)  is  in  accord  with  the  A.  I.  E.  E.  STANDARDIZATION  RULE,  Sec.  60, 
hence,  is  utilized  herein. 

EXAMPLE. — In  Fig.  41,  the  sum  of  the  individual  maximum  demands 
of  the  six  component  loads,  as  observed  from  the  maximum-demand 
meters,  MiM*  etc.,  is:  612  +  420  +  516  +  310  +  118  +  625  =  2,601 
watts  =  2.601  kw.  The  maximum  demand  of  the  whole  group  as  in- 
dicated on  the  maximum-demand  indicator  Mr,  is  only  0.86  kw.  because 
the  maximum  demands  of  the  consumers  did  not  all  occur  at  the  same 
time.  Then: 


Diversity  factor  = 


sum  of  ind.  max.  dem. 


2.601 
0.86 


3.02. 


max.    dem.    of   entire    group 

Therefore,  the  diversity  factor  between  the  six  consumers  of  Fig.  41  and 
the  supply  main  AB  is  3.02. 


Buildings 


Sum  of  Maximum  Demands 


•To  •Staff on 
'  612  +4Zdi-5l6+3IOi-ll8+6Z5-?60IWafh-2£OlKw 


Diversity  Factor 


Sum  of  Individual  Maximum  Demands  of  Components  2. 601 , 


-302 


Maximum  Demand  of  Whole  Group  '0.86  "" 

FIG.  41. — Illustrating   the   meaning  and   computation   of   diversity  factor. 

EXAMPLE. — The  maximum  demand  of  the  power  load  A  (Fig.  39), 
during  the  typical  24-hr,  period  shown,  is  290  kw.  The  maximum 
demand  of  the  lighting  load  B,  is  about  the  same,  or  270  kw.  But  the 
maximum  demand  of  the  combined  loads  is,  as  shown  at  C,  about  420 
kw.  What  is  the  diversity  factor  for  these  loads?  SOLUTION. — Sub- 
stitute in  equation  (10) :  div.  fac.  =  (sum  of  ind.  max.  dem.)  -f-  (max. 
dem.  of  entire  group)  =  (290  +  270)  -=-  420  =  560  -J-  420  =  1.33.  Hence, 
the  diversity  factor  in  this  case  is  1.33. 

ILLUSTRATIVE  EXAMPLE. — In  Fig.  42  is  diagrammed  an  imaginary 
case  where  four  consumers  1,  2,  3,  and  4  are  supplied  with  electric 
service  from  the  primary  main  AB  through  four  transformers.  The  load 
graph  for  each  of  these  four  consumers  is  shown  in  Fig.  43,  from  which  it 
is  evident  that  their  maximum  demands  are  respectively  375,  425,  450 
and  400  kw.;  the  maximum-demand  indicators  Mi,  M 2,  M j  and  M^  would 
respectively  indicate  these  individual  maximum  demands. 


58 


CENTRAL  STATIONS 


[ABT.  70 


375 XlV.    ^Jrni  425  KW    r&m\ 

rrans-~sWM     Transformer^^   A.MHERST 
former 


Maximum  Demand-  650  KwJ 
400  Kw.  Transformer  o^Energ 


\  Maximum  Demand      I  (3) 
"9 


FIG.  42. — Lay-out  of  the  loads,  graphs  of  which  are  given  in  1,  2,  3  and  4 
of  Fig.  43. 


~<IOO 

imiiiiiiiiiniiw 

34567Q9/0       //Noon/2 
FIG.  43. — Graphs  indicating  the  demands  of  four  different  imaginary  loads. 


SRC.  4]          DIVERSITY  AND  DIVERSITY  FACTORS 


59 


From  Fig.  44,  in  which  are  graphically  added  the  graphs  of  1,  2,  3 
and  4,  it  is  evident  that,  because  of  the  diversity  between  the  individual 
demands,  the  maximum  demand  of  the  entire  group  is  only  650  kw. 
If  a  maximum-demand  indicator,  M 5  (Fig.  42),  were  inserted  in  the  pri- 
mary main  it  would,  for  the  period  under  consideration,  read  650  kw. 
It  follows  then  that  the  diversity  factor  between  these  four  consumers 
would  from  equation  (10)  be: 


Div.  fac. 


sum  of  ind.  max.  dem.        375  +  425  +  450  +  400 


max.  dem.  of  entire  group 


650 


=  2.54. 


NOTE  that  while  the  transformers  to  serve  the  four  loads  shown 
would  have  to  be  about  of  the  capacities  indicated  in  Fig.  42  to  prevent 
overloading,  the  group  of  consumers  would  impose  a  maximum  demand 
of  only  650  kw.  of  the  source  of  energy. 


12  AM  I        Z 
Time 


345         6         73-9/0       UNoonlZ 
FIG.  44. — Graph  of  combined  loads  1,  2,  3  and  4. 


71.  To  Determine  Diversity  Factors  it  is  necessary  to  take 
readings  of  the  maximum  demands  on  the  different  components 
of  the  systems  under  consideration.  For  most  accurate  re- 
sults it  is  necessary  to  use  maximum-demand  indicators, 
which  usually  means  that  the  service  conductors  of  each  con- 
sumer involved  in  the  study  must  be  equipped  with  a  maxi- 
mum-demand meter.  Frequently  maximum-demand  meters 
are  used  on  the  consumers  services  in  studies  of  this  sort. 
Then,  to  obtain  the  equivalent  watts  demand,  the  maximum 
ampere  demand  imposed  is  multiplied  by  the  normal  voltage 
of  the  circuit,  it  being  assumed  that  this  voltage  remains 
constant.  H.  B.  Gear  of  the  Chicago  Edison  Company  has 
made  important  studies  of  diversity,  some  of  which  are  out- 
lined in  detail  in  his  book  ELECTRIC  CENTRAL  STATION 
SYSTEMS.  Most  of  the  demand-factor  values  recited  herein 
are  based  on  his  observations. 


60 


CENTRAL  STATIONS 


[ART.  72 


72.  There  May  be  Several  Different  Diversity  Factors 
Applying  to  the  Components  of  a  Generation  and  Distribution 
System  as  illustrated  in  Fig.  45  and  in  Table  74.  Thus,  there 
may  be  a  factor  indicating: 

1.  The  diversity  among  the  demands  of  the  different 
consumers. 


Elements 
of 
Distribution 
System 

Diversity  Factor 

Residence 
Lighting 

Lighting 

ll 

11 

A     Among 
Consumers 

336 

1.46 

1:44 

- 

B      Among 
.  Transformers 

1.30 

130 

1.35 

1.15 

G     Among 
Feeders 

X/5 

US 

US 

US 

D     Among 
SubStahons 

1.10 

1.10 

1.10 

LIO\ 

'Transmission  Line-. 


Substation 


Transmission 
Line 


^--Generating  Station 


FIG.  45. — Illustrating  diversity  of  demand  among  different  components  of  a 
distribution  system. 

2.  The  diversity  among  the  demands  of  the  transformers  in 
a  group. 

3.  The    diversity    among    the    demands    imposed  by  the 
different  feeders  on  a  sub-station. 

4.  The    diversity    among   the    demands   imposed   by   the 
different  sub-stations  on  the  generating  station  which  serves 
them. 

5.  The  diversity  among  the  demands  of  the  different  classes 
of  consumers — as  diagrammed  in  Fig.  40. 


SEC.  4]          DIVERSITY  AND  DIVERSITY  FACTORS  61 

73.  Diversity-factor  Values  Are  in  a  Measure  Determined 
by  Local  Conditions. — The  characteristics  and  habits  of  the 
people  in  a  community  will  affect  the  values  of  the  diversity 
factors  applying  to  it.     For  a  rural  city  the  diversity  factors 
will  be  somewhat  different  from  those  for  a  metropolis.     The 
factors  for  a  Southern  town  will  be  somewhat  different  from 
those  for  one  in  the  North.     Where  the  power  load  prepon- 
derates over  the  lighting,  the  overall  diversity  factor  will  be 
different  from  that  where  the  opposite  condition  holds.     Fur- 
thermore, the  values  of  the  diversity  factors  between  certain 
of  the  components  of  a  system  may  be  determined  in  a  meas- 
ure by  the  layout  of  the  system  itself,  as  explained  in  a  follow- 
ing paragraph  under  residence-lighting  transformers.     Hence, 
it  is  obvious  that  it  is  practically  impossible  to  predict  with 
accuracy  the  diversity  factor  that  will  apply  for  a  given  set 
of  conditions  unless  one  is  already  familiar  with  the  diversity 
factor  which  has  been  ascertained  by  observation  and  test  for 
like  conditions.     However,  the  factors  which  are  suggested 
below  and   given  in  Table  75  are,  probably,  fairly  typical. 
They  may  ordinarily  be  used  without  great  error  in  estimating 
situations  similar  to  those  to  which  they  apply  specifically. 

74.  The  Diversity  of  Demand  Between  Residence-light- 
ing Consumers  is  usually,  where  a  dozen  or  so  consumers  are 
involved,  represented  by  a  factor  of  about  3.4.     In  one  block 
in  Chicago,  which  was  supplied  by  a  single  transformer  and 
in  which  there  were  34  consumers  it  was  found*  that  the  sum 
of  the  consumer's  maximum  demands  was  12  kw.,  while  the 
maximum  demand  of  the  group  was  3.6  kw.     This  gave  a 
diversity  factor  of:  12  -f-  3.6  =  3.33.     In  another  block,  where 
there  were  185  consumers,  the  total  of  the  individual  con- 
sumers' maximum  demands  was  68  kw.     The  maximum  de- 
mand imposed  on  the  transformer  serving  the  block  was  20 
kw.     Hence,    the    diversity    among    these   consumers   was: 
68  -s-  20  =  3.40.     The   factor   indicating   the    diversity   be- 
tween the  demands  of  residence  consumers  is  quite  large  be- 
cause a  residence  may  be  drawing  a  considerable  load  one 
evening   and   none   at   all   the   next.     Furthermore,    if   one 


62 


CENTRAL  STATIONS 


[ART.  75 


residence  in  a  group,  possibly  because  of  some  social  function, 
is  imposing  a  relatively  large  demand,  other  residences  in  the 
same  group,  may,  because  of  the  same  occasion,  be  taking  but 
little  power.  For  these  reasons  the  diversity  factor  among 
residences  is  much  greater  than  that  among  general  power 
consumers.  See  also  the  values  given  in  Fig.  45  and  Table  75. 
75.  Diversity  Factors  for  a  Central-station  Distributing 
System. — These  data*  are  based  on  observations  made  in 
Chicago.  (The  reference  letters  A,  B,  C,  etc.,  in  the  first 
column  refer  to  Fig.  45.) 


Elements  of  distribution  system 

Diversity  factors 

Residence 
lighting 

Commercial 
lighting 

General 
power 

Large 

users 

A  Among  consumers.           .  .    . 

3.36 

1.46 

1.44 

B  Among  transformers  

1.30 

1.30 

1.35 

1.15 

C  Among  feeders  

1.15 

1.15 

1.15 

1.15 

D  Among  sub-stat/ons  

1.10 

1.10 

1.10 

1.10 

E  Consumer  to  transformer...  . 

3.36 

1.46 

1.44 



F  Consumer  to  feeder 

4.35 

1.91 

1.95 

1.15 

G  Consumer  to  sub-station  .... 

5.00 

2.19 

2.24     j     1.32 

H  Consumer  to  generator  

5.53 

2.41 

2.45 

1.45 

76.  The  Diversity  Among  the  Demands  of  Commercial 
Lighting  Consumers  is  not,  experience  shows,  nearly  so  pro- 
nounced as  with  residences.  The  reason  for  this  is  that  com- 
mercial lighting — in  the  factories  and  stores — is  ordinarily 
used  about  the  same  hours  in  the  day  and  about  the  same  days 
in  the  week  by  one  business  concern  as  by  another.  The 
result  is  that  the  diversity  factor  between  a  number  of  these 
consumers  is  usually  relatively  low,  possibly  in  the  neighbor- 

*  See  H.  B.  Gear  in  the  STANDARD  HANDBOOK. 


SEC.  4]          DIVERSITY  AND  DIVERSITY  FACTORS 


63 


hood  of  1.4.  That  the  factor  is  this  large  is  because  some 
commercial  consumers,  department  stores  for  instance,  use 
most  of  the  light  in  the  late  afternoon  or  early  in  the  even- 
ing, while  other  concerns,  hotels  for  example,  use  most  of  their 
light  in  the  evenings.  See  Fig.  40  and  Table  75. 

77.  The  Diversity  Among  Demands  Imposed  on  the  Mains 
by  Lighting  Transformers  for  residence  and  commercial  service 
may,  for  well-designed  distribution  systems,  be  represented 
probably  by  an  average  factor  of  from  1.30  to  1.35.  If  a  large 
number  of  small  transformers  are  used  to  serve  a  load,  in- 
stead of  a  few  large  ones,  the  resulting  diversity  factor 

Maximum  Demand  Meters    ...        .^Consumer's  Maximum  Demands-. 
,  (Each  Indicates  IKw.  Max.  Pem.)'\^'  I  \A        /  Kw.  Each 


JKw.  Transformers 


3Kw.  Transformer 

I  Ma 


•imum  Demand' 
2      -Meters- 
bOOKw.Max.Dem. 


Maximum  Demand  Meters "     '~"~:** -Consumers  Maximum  Demands 

(Each  Indicates  IKw.  Max.Dem)  I  Kw.  Each 

I-  20-lKw  Transformers  n~2-3Kw.  Transformers 

FIG.  46. — Showing  how  the  diversity  factor  between  transformers  may  differ 
with  the  arrangement  of  the  transformers. 

among  the  transformers  will  probably  be  greater  than  1.30. 
The  reason  for  this  is  that  if  a  large  number  of  small  trans- 
formers are  installed,  the  sum  of  the  individual  maximum 
demands  imposed  by  a  large  number  of  small  transformers 
will  be  greater  than  the  sum  of  the  individual  demands  im- 
posed by  a  few  large  transformers  serving  the  same  load,  the 
combined  maximum  demand  of  the  group  being  the  same  in 
each  case.  The  following  example  illustrates  this  principle. 

EXAMPLE. — Consider  the  imaginary  condition  illustrated  in  Fig.  46, 
where  there  are  20  consumers,  each  having  a  maximum  demand  of  1 
kw.  as  indicated  by  the  maximum  demand  meters  MI.  Assume  that 
the  maximum  demand  of  the  group  of  20  consumers  is  6  kw.  as  indicated 
by  meters  MI.  Then,  if  twenty  1-kw.  transformers  are  used  to  serve 


64  CENTRAL  STATIONS  [ART.  78 

the  load,  as  at  7,  the  diversity  factor  between  transformers  is:  20  kw. 
-=-  6  kw.  =  3.33. 

Assume  that  these  20  customers  are  to  be  served  by  only  two  trans- 
formers each  feeding  10  consumers  as  at  //.  Also  assume  that  the 
same  diversity  factor,  3.33,  would  obtain  between  consumers.  Then  the 
maximum  demand  imposed  by  transformers  /i  and  I  2  would  each  be: 
10  kw.  -r  3.33  =  3  kw.  The  sum  of  their  maximum  demands  would 
be:  3  +  3  =6  kw.  Hence,  the  diversity  factor  between  these  two 
transformers  would  be:  6  kw.  -£•  6  kw.  =  1.00.  These  conditions  are, 
obviously,  imaginary  but  the  example  indicates  how  diversity  is  deter- 
mined to  some  extent  by  the  arrangement  and  capacities  of  the  com- 
ponent equipment. 

NOTE  that  in  the  example  just  given,  the  economy  in  transformer 
capacity  resulting  from  the  grouping  of  a  relatively  large  number  of 
consumers  on  one  transformer.  In  Case  /  twenty  1-kw.  transformers 
are  required  while  in  Case  II,  two  3-kw.  transformers  will  handle  the 
load. 

The  average  diversity  factor  among  residence-lighting  trans- 
formers in  a  number  of  Minnesota  cities  is  1.60.*  This 
rather  high  value  is  attributed  to  the  fact  that  probably  an 
unnecessary  large  number  of  small  transformers  were  used. 

NOTE.  —  If  the  sum  of  the  individual  maximum  demands  of  a  group 
of  transformers  be  divided  by  the  diversity  factor  among  transformers, 
for  the  condition  under  consideration,  the  resulting  value  will  be  the 
maximum  demand  which  the  group  imposes  on  the  feeder  or  line  serving 
it.  The  examples  which  follow  illustrate  this  proposition. 

78.  To  Determine  the  Maximum  Demand  That  Will  be 
Imposed  on  Any  Transformer,  Feeder  or  Station,  when  the 
connected  load,  demand  and  diversity  factors  are  known: 
Multiply  the  connected  load  by  the  demand  factor  and  divide 
the  product  by  the  diversity  factor.  That  is: 


(13)  Max.  dem.  of  entire  group  =  (^n.^ad)  X  (dem.  fact.\ 

diversity  factor 

The  following  actual  example  recited  by  P.  J.  Nilsenf  il- 
lustrates a  practical  utilization  of  this  rule,  and  indicates  the 
economics  which  may  be  effected  through  its  application. 

ILLUSTRATIVE   EXAMPLE.—  The  utility  operated  in   a  town   of  800 
inhabitants.    Energy  was  purchased  at  13,200  volts  for  6.5  cts.  per 
•  W.  T.  Ryan. 
t  Electrical  Renew,  Aug.  5,  1916,  p.  230. 


SEC.  4] 


DIVERSITY  AND  DIVERSITY  FACTORS 


65 


kw.-hr.  A  50-kva.  outdoor-transformeT  sub-station  reduces  the  voltage 
to  2,300  for  distribution  to  the  lighting  load  detailed  below  in  Table  A. 
Before  the  hereinafter -described  changes  in  transformer  capacities  were 
effected  the  distribution  energy  losses  (energy  lost  and  unaccounted  for) 
were  equal  to  one-half  the  energy  sold;  the  distribution-loss  factor  was 
50  per  cent. 

It  is  evident  from  a  consideration  of  following  Table  A  that  the 
original  transformer  capacity  was  much  too  large.  The  sub-station 
transformer  capacity  (50  kva.)  was  twice  as  great  as  the  total  distribution 
transformer  and  larger  than  the  connected  load. 


TABLE   A. — CONNECTED   LOADS   AND   THE    TKANSFORMER    CAPACITIES 
ORIGINALLY  EMPLOYED  TO  SERVE  THEM 


Transformer  number..  . 

1 

2 

3 

4 

5 

Total 

Rating  in  kva  

5 

10 

3 

3 

5 

26 

Connected-load  kva. 
Residences  

6.48 

9.92 

6.17 

10.15 

0 

32.72 

Stores  

6.00 

0 

5.50 

0 

0 

11.50 

Streets 

o 

o 

o 

o 

4  80 

4  80 

Total  .           

12.48 

9.92 

11.67 

10.15 

4.80 

49  02 

For  a  53-day  period  during  the  winter  the  energy  purchased  was  2,500 
kw.-hr.,  while  the  energy  sold  was  only  1,600  kw.-hr.,  which  left  900 
kw.-hr.  unaccounted  for.  Simple  calculations  disclosed  that  the  sum  of 
the  transformer-core  losses  and  of  the  meter  potential-coil  losses  for  the 
53-day  period  totaled  738  kw.-hr.  Only  162  kw.-hr.  then  remained  un- 
accounted for.  This  was  probably  due  to  losses  in  the  transmission  and 
distribution  line  wires  which  losses  were  not  estimated. 

By  applying  the  demand  and  diversity  factors  specified  in  Table  B 
to  the  connected-load  values  of  Table  A  the  logical  "proposed"  trans- 
former capacities  given  in  column  IV,  Table  C  were  computed.  Thus, 
considering  only  the  maximum  demand  imposed  by  the  residence-lighting 
load  (Table  A)  on  transformer  No.  1  and  substituting  in  equation  (13): 


Max.  dem.  of  entire  group 


(con.  load)   X  (dem.  fac.)       6.48  X  0.45 


diversity  factor 


3.57 
0.817  kva. 


Then,  for  the  store-lighting  load: 

(con.  load)   X  (dem.  fac.)       6.00  X  0.75 

Max.  dem.  of  entire  group  =  —   — — — — — =  — 

diversity  factor  1.54 

-  2.92  kva. 


66 


CENTRAL  STATIONS 


[ART.  78 


But  since  there  is  a  diversity  between  the  demands  of  store  and 
residence-lighting  loads,  the  maximum  demand  imposed  by  these  two 
loads  on  the  transformer  which  serves  them  would  be: 


Max.  dem.  of  entire  group 


sum  of  ind.  max.  dem. 
diversity  factor 


0.817  +  2.92 


1.18 
3.16  kva. 


Hence,  the  estimated  maximum  demand  imposed  on  transformer  No.  1 
would  be  3.16  kva.  (column  //)  and  obviously  a  3-kva.  (column  IV) 
transformer  would  be  of  ample  capacity  to  handle  this  load.  The  other 
"Simultaneous  Maximum  Demands"  given  in  column  III  of  Table  C 
were  computed  by  a  process  similar  to  that  above  outlined. 

To  obtain  the  maximum  demand  imposed  on  the  sub-station  trans- 
former, the  sum  of  the  individual  maximum  demands  of  all  of  the  dis- 
tributing transformers  (column  77,  of  Table  C)  should  be  divided  by  the 
diversity  factor  between  these  transformers  and  the  sub-station  (Table 
B)  thus: 


TABLE  B. — DEMAND  AND  DIVERSITY  FACTORS  EMPLOYED 

The  diversity  factors  indicated  below  are  probably  somewhat  larger 
than  those  ordinarily  employed  under  similar  conditions.  However, 
their  use  is  probably  justified  in  this  instance  inasmuch  as  McNilsen 
advises  that  the  average  winter  month  consumption  per  consumer  in  this 
town  was  only  5.83  kw.-hr. 


Residences 

Stores 

Consumers'  demand  factor  

0.55 

0.75 

Group  diversity  factor: 
Consumer  to  transformer 

3  57 

1  54 

Between  stores  and  residences  .  . 
Transformers  to  substation  .... 

::::: 

1.18     |         
1.33              

SBC.  4]          DIVERSITY  AND  DIVERSITY  FACTORS 


67 


TABLE  C. — SHOWING  ESTIMATED,  SIMULTANEOUS  MAXIMUM  DEMANDS, 

NEW  TRANSFORMER  CAPACITIES  BASED  THEREON  AND  SAVINGS 

RESULTING  THROUGH  THE  USE  OF  THESE  NEW  CAPACITIES 


I 

Transformer 
number 

II 

Simultane- 
ous   de- 
mand, kva. 

in 

Rating 
present, 
kva. 

IV 
Proposed 
kva. 

V 
Yearly  saving 
in  core  losses 

VI 

Saving   in  in- 
vestment 

I 

1             !     3.16 

5 

3.0 

$5.  12  or  21 

$27.  00  or  44 

] 

per  cent. 

per  cent. 

2 

1.25 

10 

7.5* 

$6.  27  or  16 

$12.  25  or  16 

per  cent. 

per  cent. 

3 

3.03 

3 

3.0 

None 

None 

4 

1.28 

3 

1.5 

$6.  27  or  32 

$10.  20  or  29 

per  cent. 

per  cent. 

5 

4.80 

5 

5.0 

None 

None 

Sub-station  .  .  . 

11.36 

50 

15.0 

$81  .  10  or  70 

$165.  68  or  61 

| 

per  cent. 

per  cent. 

Total 

76 

35.0 

$98  76  or  42  $215  13  or  40 

per  cent.    |     per  cent. 

*  A  7.5-kw.  transformer  provided  here  to  take  care  of  a  possible  7.5- 
h.p.  day  load.     Not  necessary  to  provide  for  future  growth. 


Max.  dem.  of  entire  group 


sum  of  ind.  max.  dem. 

diversity  factor 
3.16  +  1.25  +  3.03  +  1.28 
1.33 


6.56  kva. 


Therefore,  the  simultaneous  maximum  demand  imposed  on  the  sub- 
station transformer  would  be:  6.56  +  4.8  =  11.36  kva.  (column  II, 
Table  C)  and  a  15-kva.  transformer  (column  IV,  D)  would  be  of  ample 
capacity  to  carry  the  entire  load  and  provide  for  some  growth. 

It  is  evident  from  Table  A  that  the  changes  resulted  in  a  decrease  in 
aggregate  transformer  rating  from  76  to  35  kw.  or  54  per  cent.  This 
represents  a  saving  of  approximately  40  per  cent,  in  transformer  invest- 
ment. The  change  also  resulted  in  a  decrease  of  about  42  per  cent,  in 
core  losses  or,  at  6.5  cts.  per  kw.-hr.,  $98.76  per  year  operating  expense. 
The  sub-station  transformer  core  loss  could  be  further  reduced  by  using 
a  10-kw.  transformer  from  April  1  to  Oct.  1,  and  then  installing  an  ad- 
ditional 5-kw.  unit  for  the  winter  load.  However,  the  increase  in  invest- 
ment and  attendance  cost  involved,  would  offset  the  saving  in  core  loss. 

EXAMPLE. — Consider  the  conditions  of  Fig.  47,  wherein  36  residence- 
lighting  consumers  are  shown.  The  connected  load  of  each  consumer  in 


68 


CENTRAL  STATIONS 


[ART.  78 


SEC.  4]          DIVERSITY  AND  DIVERSITY  FACTORS  69 

watts  is  indicated  at  each  building.  If  it  is  decided  to  serve  these  con- 
sumers with  three  transformers,  A,  B  and  C,  what  should  the  capacities 
of  these  transformers  be  and  what  will  be  the  maximum  demand  imposed 
on  the  primary  main  at  D?  It  will  be  assumed  that  the  consumers  de- 
mand factor  is  0.50,  that  the  diversity  factor  among  these  consumers  is 
3.35  and  that  the  diversity  factor  among  transformers  is  1.3.  SOLUTION. 
—The  connected  load  of  Group  A  is:  1,420  +  670  +  480  +  1,510  +  600 
+  1,310  +  1,480  +  515  +  910  +  680  +  1,460  +  2,520  =  13,555  watts  = 
13.6  kw.  Then  substituting  in  equation  (13): 

(con.  Id.)   X   (dem.  fac.) 

Max.  dem.  of  entire  group  =  

diversity  factor 

13.6  X  0.5 

=  — --—  =  2.00  kw. 
6.65 

Therefore,  a  2-kva.  transformer  could  be  used  at  A. 

The  connected  load  of  Group  B  is:  1,630  +  1,420  +  1,460  +  1,510  + 
450  +  550  +  11,500  +  940  +  1,500  +  420  +  1,310  +  960  +  2,560  + 
420  +  1,380  =  18,010  watts  =  18  kw.  Then  substituting  in  equation 
(13): 

(con.  Id.)    X    (dem.  fac.) 
Max.  dem.  of  ent.  group  =  — 

diversity  factor 

18  X  0.5 
^3^-2'7to- 

Therefore,  a  3-kva.  transformer  could  be  used  at  B. 

The  connected  load  of  group  C  is:  550  +  1,460  +  2,210  +  1,580  + 
320  +  2,560  +  2,400  +  1,610  +  1,240  =  13,930  watts  =  13.9  kw.  Then 
substituting  in  equation  (13): 

(con.   Id.)    X    (dem.  fac.) 

Max.  dem.  of  entire  group  = 

diversity  factor 

_  13.9  X  0.5 
3.35 

Therefore,  a  2-kva.  transformer  would  probably  suffice  at  C. 

Now  to  find  the  maximum  demand  imposed  on  primary  main  D  sub- 
stitute in  equation  (13): 

2.0  +  2.7  +  2.1       6.8 

Max.  dem.  of  entire  group  = —  =  —  =  5.2  kw. 

1.3  1.3 

Therefore,  5.2  kw.  would  be  the  maximum  demand  on  the  primary  main 
at  D.  In  actual  practice  it  would,  probably,  be  desirable  to  serve  all  the 
consumers  shown  in  Fig.  47  by  installing  one  or  two  larger  transformers, 
instead  of  using  three  relatively  small  ones  as  shown.  However,  the 
example  illustrates  the  principle  involved. 


70  CENTRAL  STATIONS  [ART.  80 

NOTE. — To  UTILIZE  THE  PRINCIPLE  OF  DIVERSITY  OP  DEMAND 
BETWEEN  LIGHTING  CONSUMERS  MOST  EFFECTIVELY,  that  is,  to  insure 
a  maximum  diversity  factor,  there  should  usually  be  a  minimum  of  from 
8  to  12  consumers  served  from  each  distributing  transformer. 

78A.  The  Diversity  of  the  Demands  Among  Feeders  ap- 
pears to  range  in  the  neighborhood  of  about  1.15  in  a  well- 
designed  system.  In  other  words,  the  maximum  demands 
on  feeders  usually  occur  almost  simultaneously. 

EXAMPLE. — If  a  generating  station  serves  two  feeders,  one  of  which 
imposes  a  maximum  demand  of  650  kw.  and  the  other  a  maximum  de- 
mand of  485  kw.,  what  will  be  the  maximum  demand  which  the  two 
will  impose  on  the  station,  it  being  assumed  that  the  diversity  factor  for 
these  feeders  is  1.15.  SOLUTION. — 

sum  of  ind.  max.  dem. 
Max.  dem.  of  entire  group  =  — 

diversity  factor 

_650+486_11135 
1.15  1.15 

Therefore,  the  maximum  demand  which  these  two  feeders  would 
impose  on  the  station  would  be  988  kw. 

79.  There  Is  a  Diversity  Among  the  Demands  of  Different 
Sub-stations  where  such  form  a  part  of  a  distribution  system. 
The  value  of  the  factor  representing  this  diversity  will  ob- 
viously be  determined  by  the  characteristics  of  the  territory 
served  by  the  sub-station.    If  one  sub-station  serves  a  resi- 
dence district  and  another  a  factory  district  the  diversity 
factor  is  liable  to  be  large.     On  the  other  hand,  the  diversity 
factor  between  the  demands  of  two  sub-stations,  both  serving 
manufacturing   communities — or    any   two    communities    of 
similar  characteristics — is  liable  to  be  small,  that  is,  in  the 
neighborhood  of  1.00. 

80.  The  Total  Diversity  Factor  for  a  System  is  equal  to  the 
product  of  the  diversity  factors  of  all  of  the  components  of  the 
system. 

EXAMPLE. — What  is  the  total  diversity  factor  for  the  residence- 
lighting  load  of  a  system  where  the  component  diversity  factors  are 
(see  Table  74)  as  follows:  Among  consumers,  3.36;  among  transformers, 
1.30;  among  feeders,  1.15;  among  substations,  1.11?  SOLUTION. — The 
product  of  these  factors  is  3.36  X  1.30  X  1.15  X  1.11  =  5.53.  That  is, 


SBC.  4]          DIVERSITY  AND  DIVERSITY  FACTORS  71 

5.53  is  the  total  diversity  factor  (note  this  value  in  the  second  column  of 
Table  75)  by  which  the  sum  of  the  individual  maximum  demands  of  light- 
ing consumers  should  be  multiplied  to  obtain  the  maximum  demand  that 
would  probably  be  imposed  by  them  on  the  generating  equipment. 
The  total  diversity  factor  in  the  Chicago  system  for  the  lighting  and 
power  load,  but  not  including  electric  railways,  is  3.2  during  the  winter 
months.* 

81.  To  Determine  the  Kilowatt  Station  Capacity  Required 
per  100  Kw.  Connected  Loads. — Divide  the  consumer's  demand 
factor  expressed  as  a  percentage  by  the  total  diversity  factor. 

EXAMPLE. — If  the  total  diversity  factor  for  a  residence-lighting  load 
is  5.53  (see  above  paragraph)  and  the  demand  factor  is  75  per  cent., 
what  kilowatt  station  capacity  will  be  required  per  100  kw.  of  lighting 
load?  SOLUTION. — 75  -f-  5.53  =  13.6.  Hence,  about  13.6-kw.  station 
capacity  would  be  required,  under  these  conditions,  per  100  kw.  con- 
nected residence-lighting  load. 

NOTE. — By  the  above  outlined  process  it  can  be  shown  that,  for  com- 
mercial-lighting loads,  about  37-kw.  station  capacity  is  necessary  per 
100  kw.  connected  and  for  general  power  loads  about  40  kw.  per  100  kw. 
connected.  In  Minneapolis!  (population  325,000)  the  ratio  of  the  maxi- 
mum demand  imposed  on  the  station  to  the  total  connected  load  is 
approximately  1  to  3,  that  is,  33-kw.  station  capacity  per  100  kw.  con- 
nected load. 

82.  One  Hundred  Per  Cent.  Minus  the  Reciprocal  of  the 
Diversity  Factor  in  Per  Cent.  Gives  the  Percentage  of  Appara- 
tus Which  May  Be  Eliminated  by  Grouping  Consumers  for 
Elements  of  a  System  Onto  One  Supply  Source. — The  follow- 
ing examples  amplify  this  statement: 

EXAMPLE. — Consider  three  individual  loads  having  maximum  demands 
of  100,  300,  200  and  600  kw.  respectively.  If  each  of  these  loads  was  to 
be  served  by  a  separate  transformer  or  station  the  aggregate  apparatus 
capacity  required  would  be:  100  +  300  +  200  +  600  =  1,200  kw. 
However,  assume  that  the  diversity  factor  between  these  loads  is  3.00. 
Then  the  maximum  demand  of  the  group  would  be  only:  1,200  -s-  3.0  = 
400  kw.  and  400  kw.  of  equipment  would  serve  the  combined  loads.  The 
saving  in  required  apparatus  would  then  be:  1,200  —  400  =  800  kw. 
That  is,  the  saving  would  be:  800  -r-  1,200  =  66.7  per  cent.  Now  also 
the  reciprocal  of  the  diversity  factor  in  this  case  is:  1  -?-  3  =  0.333  = 
33.3  per  cent.  And  100  per  cent.  -  33.3  per  cent.  =  66.7  per  cent.,  which 
verifies  the  proposition  heading  this  paragraph. 

*  H.  B.  Gear, 
t  W.  T.  Ryan. 


72  CENTRAL  STATIONS  [ART.  83 

EXAMPLE. — The  yearly  diversity*  between  the  maximum  demands  of 
lighting-and-power  and  electric-street-and  elevated-railway  loads  hi 
Chicago  for  1911  and  1912  permits,  by  combining  the  generating  appa- 
ratus, a  saving  in  apparatus  of  8.1  per  cent.  Then  the  reciprocal  of  the 
diversity  factor  would  be  100  per  cent.  -  8.1  -per  cent.  =  91.9  per  cent., 
that  is  0.919.  The  diversity  factor  would  then  be  1  -5-  0.919  =  1.09. 

83.  The  Importance  of  Diversity  as  a  Factor  in  Plant 
Design  can  readily  be  appreciated  from  a  consideration  of 
the  preceding  information.  A  distributing  or  generating 
plant  must  be  designed  largely  on  the  basis  of  the  maximum 
demand  that  will  be  imposed  on  it.  Therefore,  if  the  designer 
of  a  new  installation  is  familiar  with  the  diversity  factors 
that  are  liable  to  obtain  for  the  conditions  under  which  his 
system  will  operate  he  can  readily  determine  the  required 
capacities  for  the  members  of  the  system  by  applying  suitable 
demand  and  diversity  values.  As  hereinbefore  suggested, 
diversity  of  demand  is  of  importance  in  the  establishment  of 
rates  for  central-station  service.  It  is  a  fact  that  in  many 
cases  the  capacity  of  a  generating  station,  hence,  the  invest- 
ment, is  largely  determined  by  the  peak  lighting  load  in  the 
evening.  The  station  apparatus  must  be  large  enough  to 
handle  this  lighting  load.  However,  during  the  day  a  con- 
siderable portion  of  the  station  equipment  required  to  serve 
this  lighting  load  is  utilized  for  supplying  the  power  load. 
Obviously,  if  the  maximum  demands  of  the  power  and  lighting 
loads  were  coincident  the  station  capacity  and  investment 
would  have  to  be  much  greater  than  is  now  actually  necessary. 

NOTE. — Considering  the  situation  in  this  light,  the  energy  thus  sup- 
plied for  power  during  the  day  time  is  somewhat  of  the  nature  of  a  by- 
product and  can,  therefore,  be  sold  at  a  correspondingly  lower  rate  than 
can  energy  for  lighting  service. 

*  Samuel  Insull  in  the  STANDARD  HANDBOOK. 


SECTION  5 

LOAD  FACTOR,  PLANT  FACTOR  AND  CONNECTED- 
LOAD  FACTOR 

84.  The  Load  Factor  of  a  Machine,  Plant  or  System  is 

"the  ratio  of  the  average  power  to  the  maximum  power  during 
a  certain  period  of  time.  The  average  power  is  taken  over  a 
certain  period  of  time  such  as  a  day,  a  month  or  a  year  and 
the  maximum  power  is  taken  as  the  average  over  a  short 
interval  of  the  maximum  load  within  that  period.  In  each 
case,  the  interval  of  maximum  load  and  the  period  over  which 
the  average  power  is  taken  should  be  definitely  specified, 
such  as  'half-hour  monthly'  load  factor.  The  proper  interval 
and  period  are  usually  dependent  upon  local  conditions  and 
upon  the  purpose  for  which  the  load  factor  is  to  be  used." 

85.  Load  Factors  Are  Expressed  as  Percentages. — The 
"average  power"  may  be  either  that  generated  or  consumed, 
depending  on  whether  the  equipment  under  consideration  is, 
respectively    (1)   generating  or  delivering,  or  (2)  receiving  or 
consuming  equipment.    The  "maximum  power  averaged  over 
a  short  interval"  is  the  maximum  demand. 

NOTE.— The  term  "half -hour  monthly  load  factor"  used  in  Art.  84 
means  that  the  maximum  demand  is  based  on  a  half-hour  time  interval 
and  the  power  load  is  averaged  over  a  month. 

86.  The  Formulas  for  Load  Factor  may  (if  the  term  "maxi- 
mum demand"  which  is  really  implied,  be  substituted  for 
"maximum  power"  in  the  definition  in  Art.  84)  be  written 
thus: 

,..,  T      ,,    .  average  power 

(14)  Load  factor  =  : — 

maximum  demand 

(15)  Average  power  =  (load  factor)  X  (maximum  demand) 

,,„,„,.  ,       average  power 

(16)  Maximum  demand  =  — = — ,  ,    M 

load  factor. 

*  A.  I.  E.  E.  STANDARDIZATION  RULES,  revised  June  28,  1916,  Sec.  55. 

73 


74 


CENTRAL  STATIONS 


[ART.  87 


EXAMPLE. — In  the  central  station  serving  a  certain  city  of  8,000  in- 
habitants the  peak  load  or  30-min. -interval  maximum  demand  for  the 
year  1915  was  580  kw.  and  the  average  power  232  kw.  What  was  the 
30-min.,  annual  load  factor  for  that  year?  SOLUTION. — Substituting  in 
the  above  formula  (14)  load  factor  =  (average  power)  -H  (maximum 
demand)  =  232  -T-  580  =  0.40  =  40  per  cent.  Hence,  the  30-min.  an- 
nual load  factor  for  this  station  was  40  per  cent,  for  the  year  1915.  Figs. 
48  and  49  show  other  examples. 


MaximumDemand'SOOKw.) 


Maximum  Demand -HOOK*. , 


I -Load  Factor 


£.-545% 


FIG.  48. — Two  graphs  of  twelve-hour  loads  the  load  factor  of  each  of  which 
is  54.5  per  cent. 


\ 

II 

.       ...., 

Load  F 

»c/or  - 

ISOOKw.    , 
5000Ktv.~J 

0% 

\ 

J 

\ 

/ 

T 

/      1 

-,4,-- 

^eroye 

Load  ^ 

f 

..]TlTfr 

'< 

iTttrrT 

§ 

K 

u 

[ 

1 

NJ.6    78    9  10  II  12    1    2    3456    7    8  9   10  II  12   1 

2    3   4    5  JS 

FIG.  49. — Load  graph  for  an  average  twenty-four  hour  working  day.     Load 
factor  is  30  per  cent. 

87.  The  Real  Significance  of  a  Load  Factor  is  this:  It  affords 
an  index  as  to  the  proportion  of  the  whole  time  that  the 
machine,  plant  or  system  to  which  it  applies  is  being  worked 
at  its  full  capacity.  The  machine,  plant  or  system  must  be 
so  selected  or  designed  that  it  will  handle  the  maximum  power 
demand  that  will  be  imposed  on  it.  But  it  is  seldom,  because 


SEC.  5]    PLANT  FACTOR  AND  CONNECTED-LOAD  FACTOR  75 

of  the  general  nature  of  things,  that  any  equipment  will  have 
the  maximum  demand  which  it  can  handle  imposed  on  it 
during  all  of  the  8,760  hr.  of  a  year. 

But  whether  the  equipment  is  unloaded  or  fully  loaded 
there  are  certain  fixed  charges  (interest,  depreciation,  taxes, 
insurance,  standby  costs,  and  the  like)  which  are  adding  up 
continually.  That  is,  any  equipment  is  costing  its  owner 
money  whether  it  is  producing  or  idle.  Now  during  the  hours 
that  the  equipment  is  well  or  fully  loaded  it  is  earning  more 
money  than  it  is  spending,  hence,  nets  a  profit.  The  more 
nearly  fully  loaded  it  is  the  more  money — net — it  is  earning. 
Hence,  it  follows  from  an  economic  standpoint  that  it  is  de- 
sirable to  keep  all  equipment  as  near  fully  loaded  as  possible 
during  all  of  the  hours  of  each  year — that  is,  it  is  desirable  to 
obtain  and  maintain  a  high  load  factor.  The  graphs  of  Figs. 
50  and  51,  discussed  in  other  articles,  illustrate  this  principle. 

EXAMPLE. — If  a  machine  has  imposed  on  it  exactly  the  same  power 
load  during  all  of  the  8,760  hr.  of  a  year,  then  obviously  the  average 
power  load  is  equal  to  the  maximum  demand  and  then  the  annual  load 
factor  for  that  year  would  be  100  per  cent. 

EXAMPLE. — If  a  machine  has  imposed  on  it  1,000  kw.  half  of  the  time 
and  no  load  at  all  the  other  hah0  of  the  time,  then  the  average  load  will 
be  500  kw.  The  maximum  load  is  1,000  kw.  Hence,  the  load  factor  is 
500  -^  1,000  =  0.50  =  50  per  cent.  A  load  factor  of  50  per  cent,  then 
implies  that  the  equipment  to  which  it  relates  is  producing  to  the  extent 
of  only  half  of  its  ability. 

NOTE. — It  is  obvious  then  that  a  load  factor  denotes  the  percentage  of 
the  whole  time  which  the  equipment  is  idle,  it  being  assumed  that  the  equip- 
ment is  just  capable  of  handling  the  maximum  demand.  Load-factor 
values  are  used,  principally,  to  determine  the  average  power  (or  indirectly 
energy  expenditure)  of  an  installation  when  the  maximum  demand  is 
known — or  to  obtain  the  maximum  demand  when  the  average  power  or 
energy  consumption  is  known. 

88.  The  Effect  of  Increased  Diversity  of  Demand  Is  to  In- 
crease Load  Factor  almost  in  direct  proportion  to  the  increase 
of  diversity  factor.  Thus,  H.  B.  Gear  states  that,  in  Chicago, 
the  load  factor  of  residence  consumers  individually  is  only 
7  per  cent.,  while  in  groups  it  is  about  23  per  cent.  The  group 
load  factor  of  commercial-lighting  consumers  is  about  16  per 
cent,  and  for  general  power  users  is  about  17  per  cent.  How- 


76 


CENTRAL  STATIONS 


IABT.  89 


ever,  when  these  three  classes  of  consumers  are  combined,  the 
load  factor  of  the  load  which  they  impose  on  the  station  is 
about  35  per  cent,  during  the  winter  months. 

89.  The  Effect  of  Load  Factor  on  Central-station  Rates  is 
a  feature  that  should  be  understood.  As  the  load  factor  de- 
creases, the  cost  of  supplying  energy  must  necessarily  increase. 


15 
14 
1.3 

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ki0 

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^Q8 
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\ 

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0.2 
0.1 

0 

irf— 

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(25 

r° 

7dF 

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7      i 

1      15   20  25  30  35  40   45  50  55  60  65   70    75  SO  85  90  95  100 

Percent  Load  Factor 

FIG.  50. — Graph  illustrating  the  increase  in  fixed  charges  per  kilowatt 
hour  as  the  load  factor  increases.  (Based  on  a  plant  of  100  kw.  maximum 
load  costing  $10,000  with  fixed  charges  at  12  per  cent.) 

A  central  station  or  utility  plant  must  have  sufficient  capacity 
so  that  it  can  at  any  time  supply  the  maximum  demand  of  the 
system  which  it  serves.  Usually,  the  lighting  load  in  the  even- 
ing determines  the  maximum  demand  and,  therefore,  it  is 
only  for  a  few  hours  in  the  evening  that  the  equipment  is  earn- 
ing all  the  money  which  it  is  capable  of  earning.  During  cer- 
tain "off-peak"  hours  the  equipment  may  be  relatively  idle. 
Therefore,  when  "off-peak"  loads  can  be  obtained,  they  are 
somewhat  in  the  nature  of  a  by-product.  But  they  tend  to 
increase  the  total  load  factor  of  the  system  and  thereby  de- 
crease the  average  cost  per  unit  of  energy.  Hence,  it  is  benefi- 
cial to  all  for  the  utility  company  to  seek  and  accept  such  loads 


SBC.  5]    PLANT  FACTOR  AND  CONNECTED-LOAD  FACTOR  77 

at  rates  considerably  lower  than  those  which  it  is  necessary  to 
charge  for  service  which  may  be  coincident  with  the  peak  load 
or  maximum  demand  on  the  supply  station. 


JO        40         50         60 
Percent  Load  Factor 


90       100 


Fio.  51. — Graph  (H.  G.  Stott)  showing  how  the  cost  of  generating  energy 
increases  as  the  load  factor  decreases.  Graphs  show  only  operating  and 
maintenance  costs. 

The  following  examples  illustrate  the  application  of  these  graphs:  Consider  a 
generating  plant  operating  several  units  at  an  annual  load  factor  of  30  per  cent,  and 
making  energy  for  5  mills  per  kwh.  What  would  be  the  cost  per  kwh.  on  a  60  per  cent, 
load  factor  basis?  Refer  to  graph  C1,  then  on  the  basis  of  Mr.  Stott's  experience: 
Relative  coat  per  kwh.  at  30  per  cent,  load  factor  =  1.35  relative  cost  per  kwh.  at 
'60  per  cent,  load  factor  =  1.13.  Then,  the  expected  cost  at  60  per  cent,  load  factor  = 
(1.13  -5-  1.35)  X  5  =  4.2  mills  per  kwh. 

Now  to  illustrate  the  application  of  graphs  A,  B  or  C  which  may  be  used  for  estimating 
costs  on  the  basis  of  the  peak  or  maximum  power  load:  Consider  the  same  plant  of 
the  above  example;  the  total  annual  cost  per  kwh.  of  operating  the  plant  at  30  per  cent, 
load  factor  is:  8760  hr.  X  5  mills  =  $43.80.  But  the  annual  cost  per  peak  or  maximum 
kw,  per  year  =  $43.80  X  0.30  =  $13.14.  Now  referring  to  graph  C:Relative  cost  per 
max.  kw.  per  year  at  30  per  cent,  load  factor  =  4.05.  And,  relative  cost  per  max.  kw. 
hr.  per  year  at  60  per  cent,  load  factor  =  6.82.  Then,  the  expected  cost  per  kw.  max. 
hour,  per  year  at  60  per  cent,  load  factor  =  (6.82  -f-  4.05)  X  $13.14  =  $22.10. 

While  the  "C"  graphs  were  used  in  the  above  examples,  either  the  "A  "  or  "B" 
grapns  may  be  utilized  in  the  same  manner  for  conditions  which  justify  their  application. 

NOTE. — Fixed  charges  vary  inversely  as  the  load  factor  (see  example 
relating  to  Fig.  50).  Operating  cost*  (Fig.  51)  varies  inversely  about  as 
the  fourth  root  of  the  load  factor. 

•  H.  G.  Stott. 


78  CENTRAL  STATIONS  [ART.  91 

90.  The  Period  Over  Which  a  Load  Factor  Should  be  Reck- 
oned will  be  determined  by  the  circumstances  of  the  case. 
Where  an  ordinary  load  to  be  supplied  by  a  central  station  is 
involved,  it  is  usual  to  consider  only  an  annual  or  8,760-hr, 
load  factor — which  is  discussed  in  detail  in  another  article. 
Where  no  period  is  specified,  an  annual  load  factor  is  usually 
assumed.     Since  no-load  costs  in  an  electric  generating  station 
during  any  month  or  week  are  determined  largely  by  the  peak 
load  expected  during  the  month  or  week,*  it  is  evident  that 
load  factors  for  a  shorter  period  than  a  year  should  be  used  as 
a  basis  of  power-plant-cost  comparisons.     Possibly  the  aver- 
age daily — or  weekly — load  factor  provides  the  best  value  for 
such  comparisons. 

91.  To  Compute  the  Average  Power  or  the  Energy  Con- 
sumption Over  a  Given  Period  of  Time  the  following  formulas 
may  be  used.     To  calculate  average  power:  Divide  the  kilo- 
watt-hours energy  expended  during  the  period  by  the  number  of 
hours  in  the  period:  the  result  will  be  the  average  power,  in  kilo- 
watts, during  the  period.     That  is: 

,.,_,  kw.-hr.  expended  during  period 

(17)  Average  power  =  -  — ^r — = ^r^— 

no.  of  hr.  in  period 

and, 

(18)  no.  of  hr.  in  period  =  ^.-hr.  expended  during  period 

average  power 
also, 

(19)  kw.-hr.  expd.  dur.  period  =  (av.  power)  X  (no.  hr.  in  period). 

EXAMPLE.— Refer  to  Fig.  52.  What  was  the  average  load  on  Gener- 
ator Gi  which  operated  5,545  hr.  during  a  year  and  developed  133,086 
kw.-hr.  of  energy?  SOLUTION. — Substitute  in  equation  (17):  Av.  power 
=  (kw.-hr.  expended  during  period)  -j-  (no.  of  hr.  in  period)  =  133,080 
•4-  5,545  =  24  kw. 

EXAMPLE. — Likewise,  the  average  load  on  Generator  Gz  was:  156,000 
kw.-hr.  +  7,800  hr.  =  20  kw. 

EXAMPLE. — What  was  the  average  load  for  the  year  on  the  generating 
station  diagrammed  in  Fig.  52,  which,  as  indicated  by  watt-hour  meter 
JF3,  supplied  289,080  kw.-hr.  of  energy  during  the  entire  year  of  8,760  hr.  ? 
SOLUTION. — Substituting  in  equation  (17):  Av.  power  =  (kw.-hr.  ex- 

•  G.  I.  Rhodes. 


SEC.  5]    PLANT  FACTOR  AND  CONNECTED-LOAD  FACTOR  79 


Reads  289,080  Kwh. >Jft#fok-Totalizing  Waff  hour  Meter 


Watt  hour     \          ''Bus  Bars 
Meters 


W2 
Reads  1 56,000  Kwh. 


Operated  7800 Hr& 
f.  '.^T^Tyj^  during  the  Year  12345 


r  Operated  5,545  Mrs.  during  the  Year 

Fio.  52. — Examples  in  determining  average  loads. 


FIG.  53. — Determination  of  average  load  imposed  by  an  industrial  plant. 


Registers  234 
!  Kw.  Hr  in  10  Hrs 


Motor 
30Kw. 


5 'Arc  Lamps  at 
500  Watts- 2.5  Kw. 


Motor  13  Kw.-  • 


\Watthour 
~ 


.Pan  el  Box  for 
! Incandescent  Lamps 
5.5  Kw. 

\    Welding  Machined 
Power  Inpufsl-—'*-5  *"• 
FIG.  54. — A  capacity-factor  problem. 


80 


CENTRAL  STATIONS 


[ART.  92 


pended  during  period)  -^  (no.  of  hr.  in  period)  =  289,080  4-  8,760  = 
33  kw.  =  average  annual  load. 

EXAMPLE.—  The  watt-hour  meter  totaling  the  energy  supplied  to  a 
certain  industrial  plant  (Fig.  53)  indicated  during  a  certain  year  an 
annual  consumption  of  167,302  kw.-hr.  What  was  the  average  load  im- 
posed by  this  plant  during  this  year?  SOLUTION.—  Substituting  in  equa- 
tion (17):  Av.  power  =  (kw.-hr.  expended  during  period)  -f-  (no.  of  hr. 
in  period)  =  167,302  -^  8,760  =  19.1  kw.  That  is,  the  average  annual 
load  imposed  was  19.1  kw.  Also  see  Fig.  54,  for  another  example. 

92.  To  Determine  the  Average  Power  From  a  Load  Curve, 

either  the  graphic  method  illustrated  in  Fig.  55  and  in  the 
following  example  or  that  involving  the  use  of  a  planimeter 
(Figs.  56,  57,  58  and  59)  described  below  may  be  used.  The 
general  procedure  is  quite  similar  to  that  used  in  obtaining  the 
mean  effective  pressure  from  a  steam-engine  indicator  diagram. 


Kilowatt  Loads  at  Each  Hour<-:-~. 


A.M.  P.  M 

FIG.  55. — Illustrating    method    of    computing    maximum    demand    from    & 
load  graph. 

93.  The  Rule  for  the  Graphic  Method  of  Finding  the 
Average  Power  From  a  Load  Curve  is :  Scale  or  read  from 
the  load  graph,  the  momentary  power  expenditures  at  the  ends 
of  suitable,  equal  time  intervals  over  the  entire  time  comprehended 
by  the  graph.  Then  add  these  momentary  power-expenditure 
values  together  and  divide  their  sum  by  the  number  of  periods 
into  which  the  entire  time  was  apportioned.  The  result  will 
be  the  average  power  expenditure  or  average  load. 


SEC.  5]    PLANT  FACTOR  AND  CONNECTED-LOAD  FACTOR  81 

The  number  of  time  intervals  into  which  the  entire  time 
should  be  divided  is  determined  by  the  contour  of  the  graph 
and  by  the  degree  of  accuracy  desired.  In  general,  the  greater 
the  number  of  intervals  taken  the  more  accurate  will  be  the 
result.  However,  where  the  contour  of  the  graph  is  quite 
regular  and  it  comprehends  24  hr.  of  time,  as  in  Fig.  55,  the 
result  will  usually  be  sufficiently  accurate  for  practical  work 
if  1-hr,  intervals  are  assumed.  It  is  seldom  necessary  to  use 
intervals  smaller  than  half  an  hour  on  a  graph  comprehending 
a  24-hr,  period.  When  the  contour  of  the  graph  is  extremely 
irregular  and  comprehends  a  short  period  of  time,  it  may  be 
desirable,  to  insure  sufficiently  accurate  results,  to  use  15-min., 
5  min.  or  even  1-min.  intervals. 

EXAMPLE. — What  is  the  average  power  for  the  load  graph  shown  in 
Fig.  55?  SOLUTION. — Since  this  curve  (which  is  an  imaginary  one)  is 
quite  regular,  the  momentary  power  expenditures  were  taken  at  the  end 
of  each  1-hr,  interval.  These  are  given  above  the  graph:  30,  25,  20,  20 
kw.,  etc.  The  sum  of  all  of  these  momentary  demands  is  802  kw.  Since 
the  graph  was  divided  into  24  intervals,  average  load  =  802  •*-  24  = 
33.4  kw. 


Planimeter. 


Weight         .  Load Graph 


Drawing  Board''  -Thumb  Tacks^    Tracer  Punt 

FIG.  56. — Finding  the  average  load  from  a  load  graph  with  a  planimeter. 

94.  In  Determining  the  Average  Power  From  a  Load 
Graph  With  an  Ordinary  Polar  Planimeter  (Figs.  56  and  57) 
ascertain  the  actual  area  of  the  portion  within  the  graph — which 
represents  energy  or  kilowatt-hour — in  square  inches  by  using 
the  planimeter.  Now  divide  this  area  by  the  actual  length  of 


82 


CENTRAL  STATIONS 


[ART.  94 


the  graph  in  inches.  The  quotient  thus  obtained  will  be  the 
average  height  or  length  or  ordinate  of  the  graph,  in  inches. 
Then  find  by  scaling  along  the  ordinate  axis  of  the  graph  the 
kilowatt  equivalent  of  this  average  height.  This  kilowatt  equiva- 
lent will  be  the  average  power  in  kilowatts. 

If  the  planimeter  which  is  being  used  is  not  sufficiently  large 
to  indicate  the  area  of  the  graph  under  consideration  in  one 
sweep,  divide  the  total  area,  with  vertical  pencil  lines,  into 
three  or  four  sections,  which  need  not  be  equal;  find  the  area 


gS. . 

3*  Actual  Length  of  Original  Graph 

FIG.  57. — Example  of  the  determination  of  average  load  using  a  planimeter. 

of  each  section  and  then  add  all  of  these  partial  areas  together 
to  obtain  the  total  area.  Detailed  directions  for  using  planim- 
eters  accompany  each  instrument  or  may  be  found  in  books 
on  steam-engine  or  indicator  practice. 

EXAMPLE. — Find  the  average  power  from  the  graph  of  Fig.  57  with  a 
planimeter  and  compute  the  energy  thereby  represented.  SOLUTION. — 
By  using  the  planimeter  it  is  found  that  the  area  (ABCDEFGHIJKLMA) 
within  the  graph,  representing  energy,  is  27,547  sq.  in.  (Note  that  the 
cut  shown  is  only  about  one-third  the  size  of  the  original  graph  on  which 
this  example  was  based.)  The  length,  AL,  of  the  graph  is  found,  by  scal- 
ing, to  be  9%  in.  Now  9%  in.  =  9.625  in.  Hence,  the  average  height 
or  ordinate  of  the  graph  is:  27,547  -f-  9.625  =  2.86  in.  . 

It  will  be  found  by  scaling  along  the  ordinate  AN  that  2.86  in.  is 
equivalent  to  572  kw.,  that  is,  1  in.  =  200  kw.  Hence,  the  average 


SEC.  5]    PLANT  FACTOR  AND  CONNECTED-LOAD  FACTOR  83 

power  is  572  kw.  as  indicated  by  line  OP  which  has  been  drawn  in  on 
the  sheet.  Furthermore,  the  energy  represented  by  the  shaded  area 
within  the  graph  is:  572  kw.  X  24  hr.  =  13,728  kw.-hr. 

If  it  were  known  at  the  start  that  the  energy  represented  by  the  shaded 
portion  within  the  graph  was  13,728  kw.-hr.,  the  procedure  to  determine 
the  average  power  might  then  have  been  thus:  13,728  kw.-hr.  -5-  24  hr.  = 
572  kw.-hr.  which  is  the  average  power  or  load. 

95.  To  Determine  the  Average  Power  From  a  Load  Graph 
With  an  Amsler-Type  Polar  Planimeter  (Figs.  58  and  59)  the 
process  is  simpler  than  that  just  described  because,  with  this 
instrument,  it  is  unnecessary  to  scale  the  length — represent- 
ing time — of  the  graph.     By  performing  a  simple  subtraction 
and  division  the  average  height  (which  is  proportional  to  the 
average  power)  of  the  graph  may  be  obtained  directly  as 
outlined  under  Fig.  59. 

96.  To  Determine  the  Maximum-demand  value  for  use  in 
computing  a  load  factor,  the  most  desirable  and  accurate 
method  is  to  use  the  reading  of  a  maximum-demand  meter 
where  such  is  available.     Where  load  graphs  are  available, 
the  maximum  demand  can  be  readily  taken  from  them.     The 
record  of  a  graphic  wattmeter  is  useful  in  this  connection. 
A  load  graph  may  be  plotted  from  the  readings,  taken  at 
equidistant  time  intervals,  of  an  indicating  ammeter  or  watt- 
meter and  the  maximum  demand  may  then  be  ascertained 
from  the  graph  thus  plotted.     The  examples  herein  recited 
illustrate  the  general  methods.     Where  the  demand  factor 
applying  to  a  connected  load  of  a  certain  character  is  known, 
the  maximum  demand  can  then  be  obtained  by  multiplying 
the  connected  load  in  kilowatts  by  the  demand  factor;  the 
result  will  be  the  maximum  demand  in  kilowatts.     The  defini- 
tion of  connected  load  is  given  below.     The  following  equations 
indicate  how  this  process  may  be  utilized  in  solving  load-factor 
problems. 

NOTE. — The  maximum  demand  of  an  energy-receiving  installation  is 
sometimes,  erroneously,  assumed  as  equal  to  the  connected  load.  While 
this  assumption  may  approximate  the  facts  in  certain  isolated  instances, 
such  is  seldom  the  case.  The  value  obtained  by  dividing  average  load 
by  connected  load  does  not  give  "load  factor"  but  gives  "connected-load 
factor"  as  explained  in  a  following  article. 


84  CENTRAL  STATIONS 

Ams/er-Type  Polar Planimeter  ..  PI     (Graph BefngMeasureef 


[ART.  96 


•acing 
°ofnf 


Thumb  Tacks  ' 


Drawing  Board 

FIG.  58. — Amsler-type  polar  planimeter  being  used  in  measuring  the  area 
within  a  load  graph. 


fNeedte  Pole 

:       'Tracing  Point 


Boiler  Wheel 


Graph 


.12    2    4    6    8   10  12   2    4    6    8   10  /, 


Thumb 
Tacks 


FIG.  59. — Method  of  setting  planimeter  so  that  it  will  read  the  mean  or 
average  height  of  graph. 

The  planimeter  is  held  upside  down  and  points  P  and  Pi  are  so  adjusted  that  the  dis- 
tance between  them  is  exactly  equal  to  the  length  L  of  the  load  graph.  Then  the  arm  is 
clamped  and  the  planimeter  used  in  the  usual  way.  However,  instead  of  indicating 
the  area,  the  mean  height  of  the  graph  may  be  ascertained  from  the  readings.  Thus, 
with  one  make  of  instrument,  the  difference  between  the  readings  at  the  beginning  and 
at  the  end  of  the  operation  divided  by  0.4  will  give  the  mean  heieht  of  the  graph  in 
inches.  Example:  (Second  reading,  4,322)  —  (first  reading,  4,786)  =  0.464.  Now 
0.464  ^-  0.4  =  1.16  which  is  the  mean  height  of  the  diagram  in  inches. 


SEC.  5]    PLANT  FACTOR  AND  CONNECTED-LOAD  FACTOR  85 

97.  The  Equation  for  Computing  Load-factor  Problems  on 
the  Basis  of  Demand  Factor  and  Connected  Load  Where 
Only  One  Load  is  Under  Consideration  follow  from  the  fact 
disclosed  in  equation  (8)  that  :  Maximum  demand  =  connected 
load  X  demand  factor.  Substituting  this  expression,  which  is 
taken  from  Art.  53,  in  formula  (14)  it  follows  that: 

/™\       T     3  f    i         av-  power  average  power 

(20)       Load  factor  =  rn=  (dem.factor)  X  (con.  load}' 


(21)    Average  power  =  (dem.fac.)  X  (con.  load)  X  (load  factor)  . 


(22)  Dem.  fac.  =  ^  power 

(load  factor)  X  (con.  load) 

(23)  Connected  load  = 


(dem.  factor)  X  (load  factor) 

EXAMPLE.  —  The  connected  load  of  a  certain  theatre  is  3.2  kw.  Its 
average  power,  consumption  during  the  year  (8,760  hr.)  1915  was  0.27 
kw.  If  a  demand  factor  of  49  per  cent,  be  assumed  as  applying  to  this 
class  of  service  —  see  accompanying  Table  99  —  what  will  be  the  annual 
load  factor  for  this  installation?  SOLUTION.  —  Substitute  in  equation 
(20):  Load  factor  =  (av.  power)  H-  [(dem.  fac.)  X  (con.  toad)]  =  0.27 
-H  (0.49  X  3.2)  =  0.27  -J-  1.57  =  0.172  =  17.2  per  cent.,  which  is  the 
annual  load  factor  which  may  reasonably  be  expected  for  this  class  of 


98.  If  Several  Different  Loads  or  a  Group  of  Loads  Are 
Under  Consideration  and  there  is  a  diversity  among  their 
demands,  a  diversity  factor  may  be  introduced  into  the 
formulas.  It  can  be  shown  that,  equation  (13),  for  several 
different  loads:  Max.  dem.  —  (connected  load)  X  (demand 
factor)  -H  (diversity  factor)  .  Hence,  substituting  this  expression 
for  maximum  demand  in  equation  (20)  : 

av.  power 
(24) 


(av.  power)  X  (diversity  factor) 
~  (connected  load)  X  (dem.  factor) 

(av.  power)  X  (diversity  factor) 

(25)  Connected  load  =  jr,  —  /.    A  \  v/,  .  --  *j.    .    (• 

(load  factor)  X  (demand  factor) 

(av.  power)  X  (diversity  factor) 

(26)  Demand  factor  =*  -;T  —  .-.    J  \  -  -.  —       *  J,  .  —  -£• 

(load  factor)  X  (connected  load) 


86 


CENTRAL  STATIONS 


[ART.  99 


(load  /ac.)  X  (con,  load)  X  (dem.  /ac.) 
(27)        Av.  power  =  -  diversity  factor 


(28)    Diversity  factor  = 


(load  /ac.)  X  (con,  load)  X  (dem.  /ac.) 
average  power 


FIG.  60. — Example  in  computing  load  factor  on  the  basis  of  connected  load, 
demand  factor  and  diversity  factor. 

EXAMPLE.— What  will  be  the  probable  annual  load  factor  (Fig.  60)  of 
the  load  imposed  at  A  by  the  five  manufacturing  plants  shown,  if  the 
average  load  is  2.06  kw.  It  is  assumed  that  the  demand  factor  of  their 
loads  is  50  per  cent,  and  the  diversity  between  them  is  1.44?  SOLUTION. — 
The  total  connected  load  is:  2.2  +  8.6  +  3.2  +  4.1  +  1.7  =  19.8  kw. 
Then,  substituting  in  equation  (24):  Load  factor  =  (av.  power)  X 
(diversity  factor)  -T-  (con.  load)  X  (dem.  factor)  =  (2.06  X  1.44)  ^  (19.8 
X  0.5)  =  (2.96  -T-  9.9)  =  0.30  =  30  per  cent.  Hence,  the  load  factor  of 
the  load  imposed  at  A  is  30  per  cent. 

99.  Load  Factors,  Demand  Factors  and  Connected-load 
Factors  of  "Small"  and  "Medium"  Lighting  Customers  in 
Chicago. — The  values  in  columns  A  and  B  are  averages  from 
the  information  of  actual  tests.*  They  are  based  on  data  pre- 
sented before  a  National  Electric  Light  Association  conven- 
tion by  R.  W.  Lloyd.  The  values  in  column  C  were  obtained, 

•  Based  on  data  presented  before  a  National  Electric  Light  Association  convention  by 
E.  W.  Lloyd. 


SEC.  5]     PLANT  FACTOR  AND  CONNECTED-LOAD  FACTOR  87 

in  accordance  with  the  method  described  in  another  paragraph, 
by  multiplying  together  the  corresponding  values  of  columns 
A  and  B.  It  should  be  understood  that  while  these  values  are 
"representative,"  in  that  they  are  based  on  averages  of  ob- 
served data,  it  does  not  necessarily  follow  that  the  same  values 
will  be  obtained  under  all  conditions  for  the  loads  of  the  dif- 
ferent classes  enumerated.  However,  they  may,  ordinarily, 
be  safely  used  in  estimating  where  precise  values  applying 
to  the  particular  conditions  under  consideration  are  not  avail- 
able. All  of  the  following  values  are  expressed  in  per  cent. 
The  load  factors  and  the  connected-load  factors  are  on  an 
annual  (8,760-hr.)  basis. 


Kind  of  business 

A 

Load 
factor 

D 
Demand 
factor 

Connected- 
load  factor 

Banks 

16 

67 

11 

Churches 

12 

56 

Hotels  .  . 

24 

28 

7 

Houses  
Offices  (business)  

Offices  (professional)  
Pool  and  billiards  . 

8 
9 

7 
17 

43 

64 

64 
65 

3 

8 

4 
11 

Printers  and  engravers  
Restaurants  
Saloons 

15 
23 
21 

59 
52 
63 

9 
12 
13 

Shops  (barber)  

12 

70 

g 

Shops  (machine)  
Shops  (tailor)  
Stables  (livery)  
Stores  (book  and  stationery)  .... 

Stores  (cigar) 

9 
8 
22 
12 

17 

37 
59 
52 
66 

65 

3 
5 
11 
8 
11 

Stores  (house  furniture)  

8 

52 

4 

Stores  (dry  goods)  
Stores  (drug)  
Stores  (furniture)  

Stores  (grocery)  

0 

19 
6 

10 

77 
79 
70 

73 

6 
15 
4 

7 

Stores  (hardware)  . 

11 

40 

4 

Stores  (jewelry)  
Stores  (shoe)  •  

15 
10 

64 
67 

10 

8 

Stores  (clothing)  

7 

53 

4 

88 


CENTRAL  STATIONS 


[ART.  100 


Kind  of  business 

A 
Load 
factor 

B 
Demand 
factor 

c 

Connected- 
load  factor 

Small  hotels  and  rooming  houses. 

26 

67 

17 

10 

68 

7 

Theatres 

17 

49 

8 

12 

41 

5 

Wholesale  houses                 .... 

19 

47 

9 

Manufacturers        

10 

54 

5 

Hospitals  

13 

42 

5 

Flats... 

7 

54 

4 

100.  Load  Factors,  Demand  Factors  and  Connected-load 
Factors  of  "Large"  Combined  Power-and-light  Consumers 
in  Chicago. — These  values  are  averages  of  actual  tests.* 
Their  use  should  be  subject  to  the  restrictions  specified  in  the 
heading  of  Table  99.  All  of  the  following  values  are  expressed 
in  per  cent.  The  load  factors  and  the  connected-load  factors 
are  on  an  annual  (8,760-hr.)  basis. 


Kind  of  business 

A 
Load 
factor 

B 
Demand 
factor 

c 

Connected- 
load  factor 

Butter  and  creamery 

20 

60 

12 

Breweries  

45 

60 

27 

Brass  and  iron  beds  .  . 

20 

60 

12 

Biscuit  manufacturers  
Boots  and  shoes  
Brass  manufacturing  
Boiler  shops     . 

35 
25 

28 
18 

55 
65 
50 
45 

19 
16 
14 

8 

Can  manufacturers  
Candy  manufacturers  

30 
18 

70 
45 

21 
8 

Clothing  manufacturers 

15 

55 

8 

Clubs  (large) 

40 

85 

34 

Department  stores  (large)  
Electrical  manufacturing  

30 
25 

55 
55 

17 
14 

Express  companies 

40 

60 

24 

Electroplating  .  .  . 

25 

75 

19 

Engraving  and  printing 

19 

60 

11 

Fertilizer  manufacturing  

75 

40 

30 

*E.  W.Lloyd. 


SBC.  5]    PLANT  FACTOR  AND  CONNECTED-LOAD  FACTOR  89 


A 

Load 
factor 


B 

Demand 
factor 


Furniture  manufacturing i  I 

Foundries 15 

Forge  shops 30 

Grain  elevators :  10 

Glove  manufacturing !  25 

Grocers  (wholesale) 20 

Hotels  (small) 35 

Hotels  (large) 50 

Ice-cream  manufacturing |  45 

Jewelry  manufacturing i  18 

Laundries 25 

Machine  shops !  26 

Newspapers 20 

Packing  houses 30 

Paint,  lead  and  ink  manufacturers  23 

Paper -box  manufacturers 25 

Plumbing  and  pipe  fitting 26 

Post  offices 50 

Power  buildings 27 

Refrigeration '  50 

Railroad  depots j  50 

Pneumatic  tube :  50 

Soap  manufacturers i  25 

Seed  cleaners i  25 

Screw  manufacturers 30 

Spice  mills 20 

Saw  manufacturers 30 

Structural  steel 22 

Sheet-metal  manufacturers 18 

Stone  cutters 17 

Twine  mills 30 

Theatres 16 

Large  restaurants 50 

Small  restaurants 30 

Woolen  mills 27 

Wood-working 28 

Textile  mills...  20 


65 
75 
49 

75 
55 
55 
50 
40 
75 
50 
70 
55 
75 

75 
45 
50 
55 
30 
40 
90 
50 
90 
60 

55 
75 
55 
55 
40 
70 
55 
60 
60 
60 

70 
80 
65 
65 


CENTRAL  STATIONS 


[AKT.  102 


101.  Annual  or  Yearly  Load  Factor  is  equal  to  the  average 
power  load  over  the  entire  year  divided  by  the  maximum  power 
demand  during  that  year.  A  year  is  taken  as  having:  365 
days  X  24  hr.  =  8,760  hr. 

EXAMPLES  OP  TYPICAL  YEARLY  LOAD  FACTORS  for  central-station 
loads  are  given  by  J.  R.  Cravath  thus:  A  purely  lighting  load  in  a  small 
town  will  yield  at  the  supplying  station  a  yearly  or  8,760-hr,  load  factor 
of  less  than  20  per  cent.;  in  a  large  city  it  will  be  less  than  25  per  cent. 
By  the  addition  of  electric-motor  and  heating-appliance  loads,  these  load 
factors  have  been  improved  (see  Fig.  61)  from  year  to  year  during  the 


1301  1902  1903  1904  1905  1906  1907  1908  1909  19/0  1911    1912   1913  1914  1915 


FIG.  61.  —  Graph  showing  annual  load  factors  of  the  Commonwealth 
Edison  Co.,  Chicago.  This  indicates  how  the  load  factor  of  a  system  may  be 
improved  through  systematic  persistent  effort. 

history  of  the  central-station  industry.  A  load  factor  of  between  30  and 
35  per  cent,  is  now  common  in  the  smaller  plants  having  a  moderate 
power  load.  In  some  manufacturing  cities,  load  factors  greater  than  50 
per  cent,  have  sometimes  been  attained,  but  such  instances  are  rare. 
A  combination  of  lighting,  power  and  railway  loads  in  a  large  city  pro- 
vides a  load  factor  between  40  and  45  per  cent. 

102.  Operating  Load  Factor  is  the  ratio  of  the  average  power 
load  imposed  on  a  plant  or  by  equipment,  during  the  time  which 
the  plant  or  equipment  operates,  to  the  maximum  power  demand 
imposed  during  that  time.  Frequently,  central-station  plants 


SEC.  5]    PLANT  FACTOR  AND  CONNECTED-WAD  FACTOR  91 


in  small  cities  operate  only  during  the  night  and  industrial- 
plant  generating  stations  may  operate  only  during  the  day. 
Operating-load  factors  have  their  application  for  conditions 
such  as  these. 

EXAMPLE. — A  certain  small  central  station  in  a  town  of  750  inhabitants 
in  Iowa  operates  3,540  hr.  per  year.  The  energy  generated  during  the 
3,540  hr.  of  operation  is  15,576  kw.-hr.  The  maximum  power  demand 
or  maximum  load  is  16.9  kw.  (1)  What  is  the  annual  load  factor?  (2) 
What  is  the  operating  load  factor?  SOLUTION. — (1)  The  average  power 
or  load  during  operation  is:  15,576  kw.-hr.  -r-  3,540  hr.  =  4.4  kw.  Hence, 
operating  load  factor  =  (average  power  load  during  operation)  -5-  (maxi- 
mum demand)  =  4.4  -f-  16.9  =  0.26  =  26  per  cent.  (2)  The  average 
load  over  the  entire  year  is  15,576  kw.-hr.  -i-  8,760  hr.  =  1.775  kw. 
Hence,  the  annual  load  factor  =  (average  power  load  over  entire  year) 
•T-  (maximum  demand)  =  1.775  -5-  16.9  =  0.105  =  10.5  per  cent. 


too 


" 


01     2    3    4    5    6    7   8    9   10  II  12  13  14  15  16  17  18  13  20  21  22  23  24 
Equivalent  Hours  per  Day  Duration  of  Maximum  Demand 

0       730     1460    2190    2920  3650  4380    5110     5840   6570    7300    8030  8760 

365      1095     1825    Z5S5    3285   4015    4745    5475    6205    6335      7665    83S5 
Kilowatt  Hours  Energy  Expenditure, per  Year,  per  Kw  of  Maximum  Demand 

FIG.  62. — Graph  showing  relation  of  load  factor  to  equivalent  hours  use  of 
maximum  demand. 

103.  To  Compute  the  Energy  Delivered  or  Consumed  by 
a  Given  Installation  of  Known  Load  Factor  when  the  maximum 
demand  is  known,  reckon  the  average  power  by  using  formula 
(15)  and  then  figure  the  energy  consumption  by  applying 
formula  (19).  Where  the  maximum  demand  is  not  known 
but  where  the  connected  load  and  demand  and  diversity 
factors  are  known  equation  (21)  or  (27)  can  be  used  for 
calculating  the  average  power.  Frequently,  the  graph  of 


92  CENTRAL  STATIONS  [ART.  104 

Fig.  62  can  be  used  to  advantage  as  illustrated  in  the  following 
examples : 

EXAMPLE.— An  annual  load  factor  of  25  per  cent.  (Fig.  62)  implies  a 
6-hr,  use  per  day  of  the  maximum  demand  and  an  annual  energy  ex- 
penditure of  2,190  kw.-hr.,  per  kw.  of  maximum  demand.  Thus  if  a 
certain  installation  has  a  load  factor  of  25  per  cent,  and  its  maximum 
demand  is  42  kw.  the  annual  energy  expenditure  involved  is:  42  X  2,190 
=  91,980  kw.-hr. 

104.  A  Specific  Example  Showing  How  Fixed  Charges  per 
Kilowatt-hour  Increase  with  Decreasing  Load  Factor,*  is 

stated  graphically  in  Fig.  50.  This  is  based  on  an  assumed 
plant  having  a  maximum  capacity  of  100  kw.,  and  an  assumed 
cost  of  $10,000.  A  fixed  charge  of  12  per  cent,  is  assumed 
thus:  interest  5  per  cent.,  depreciation  5  per  cent.,  insurance 
and  taxes  2  per  cent.  The  fixed  charge  per  kilowatt-hour 
generated  may  be  determined  in  this  way: 

EXAMPLE.— The  yearly  fixed  charge  (Fig.  50)  will  be:  0.12  X  $10,000 
=  $1,200.  If  the  plant  operated  at  a  load  factor  of  100  per  cent.— 8,760 
hr.  per  year— it  would  develop:  8,760  hr.  X  100  kw.  =  876,000  kw.-hr. 
per  year.  Then  the  fixed  charge  per  kilowatt-hour  would  be:  $1,200  -=- 
876,000  kw.-hr.  =  $0.00137  =  0.137  eta.,  as  plotted  in  Fig.  50  at  A.  Now 
if  the  load  factor  is  25  per  cent.,  only  one-half  the  energy  would  be  gen- 
erated, that  is,  there  would  be  generated  8,760  hr.  X  25  kw.  =  219,000 
kw-hr.  per  year.  Then  the  fixed  charge  per  kilowatt  hour  would  be: 
$1,200  •*•  219,000  =  $0.00548  =  0.548  cts.,  as  plotted  at  B.  That  is, 
with  a  load  factor  of  25  per  cent,  the  fixed  charge  has  been  increased  four- 
fold. The  other  points  on  the  graph  may  be  determined  by  a  similar 
process. 

105.  Plant  Factor f  is  "the  ratio  of  the  average  load  to  the 
rated  capacity  of  the  power  plant,  i.e.,  the  aggregate  ratings 
of  the  generators."     That  is: 

/r.~s  average  load 

(29)  Plant  factor  =  -—-, ~. 7—1 — 

rated  capacity  of  plant 

(30)  Average  load  =  (plant  factor)  X  (rated  capacity  of  plant) 

(31)  Rated  capacity  of  plant  =    ™"*ff*l™*. 

plant  factor 


*  Albert  F.  Strouae. 

t  A.  I.  E.  E.  STANDARDIZATION  RULES,  Sec.  56. 


SEC.  5]    PLANT  FACTOR  AND  CONNECTED-LOAD  FACTOR  93 

EXAMPLE. — The  generating  equipment  in  the  central  station  shown 
in  Fig.  63  comprises  two  600-kw.  (continuous-rating),  turbo-generator 
units,  A  and  B.  If  it  is  assumed  that  the  average  power  load  imposed 
on  the  station  is  255  kw.,  what  then  is  its  plant  factor?  SOLUTION. — 
From  formula  (29):  Plant  factor  =  (average  load)  -f-  (rated  capacity 
of  plant  of  generators)  =  255  kw.  -5-  (600  kw.  +  600  kw)  =  255  4-  1,200 
=  0.212.  That  is,  the  plant  factor  is,  on  a  continuous  rating  basis,  21.2 
per  cent. 


FIG.  63. — Kalamazoo,  Mich.,  municipal  lighting  plant 

106.  There  May  Be  an  Annual  and  an  Operating  Plant 
Factor  just  as  there  may  be  an  annual  and  an  operating  load 
factor,  as  discussed  in  Art.  102.  In  fact,  plant  factor  may  be 
determined  over  any  suitable  period  of  time  just  as  can  load 
factor.  Note  that  plant  factor  applies  only  to  energy- 


94  CENTRAL  STATIONS  [ART.  107 

generating  or  delivering  apparatus  and  that  it  does  not  apply 
to  energy-consuming  apparatus.  While  the  explanatory  defi- 
nition as  given  in  the  A.  I.  E.  E.  STANDARDIZATION  RULES 
refers  specifically  to  the  "aggregate  ratings  of  the  generators," 
plant  factor  may  properly  be  computed  on  the  basis  of  the 
output  of  any  energy-delivering  plant — not  necessarily  a 
generating  plant.  Thus,  a  plant  factor  may  be  computed 
relating  to  the  output  of  a  transformer,  motor  generator, 
synchronous  converter  or  any  similar  sort  of  a  sub-station. 

107.  APlant  Factor  Does  Not  Have  aDefinite  Meaning  Unless 
the  Method  Used  in  Rating  the  Capacity  of  the  Station  is 
Specified. — The  station  may  be  rated  on  a  "normal-power- 
capacity"  basis  or  on  a  "continuous"  or  "maximum-power- 
capacity"  basis  and  the  continuous  capacity  may  be  from  25 
to  40  per  cent,  or  more  greater  than  its  normal  capacity. 
However,  since  the  "continuous"  method  of  rating  electrical 
apparatus  is,  probably,  in  most  cases  the  more  logical,  it 
should  always  be  used  where  feasible.     The  continuous  rating 
is  defined  below.     Where  no  method  of  rating  is  specified,  it 
is  logical  to  assume  that  the  continuous  method  is  implied. 

108.  The  Continuous  Rating  of  a  piece  of  electrical  ap- 
paratus is  that  rating — usually  expressed  in  horse-power  or 
kilowatts  but  sometimes  in  amperes — at  which  the  machine 
or  device  may  operate  continuously  without  its  limitations 
being  exceeded.     That  is,   without  its   becoming   so   over- 
loaded that  it  will  be  overheated  and  damaged  or  becomes 
unsafe,  inefficient  or  operate  with  a  poor  performance.     A 
continuous  rating  is  often  referred  to  as  a  maximum  rating. 

NOTE.* — "A  machine  rated  for  continuous  service  shall  be  able  to 
operate  continuously  at  its  rated  output  without  exceeding  its  limitations 
dictated  by:  (1)  Operating  temperature,  (2)  mechanical  strength,  (3)  com- 
mutation, (4)  dielectric  strength,  (5)  insulation  resistance,  (6)  efficiency,  (7) 
power  factor,  (8)  wave  shape,  and  (9)  regulation. 

EXAMPLE. — Most  types  of  electrical  machinery  may  be  given  either 
"normal"  or  "maximum"  ratings.  The  normal  rating  indicates  the 
load  which  the  machine  will  carry  continuously  and  with  a  certain  over- 
load for  a  specified  time.  The  maximum  or  continuous  rating  indicates 
the  load  which  the  machine  will  carry  continuously  but  without  any  over- 

*  A.  I.  E.  E.  STANDARDIZATION  RULES,  Sec.  281. 


SEC.  5]    PLANT  FACTOR  AND  CONNECTED-LOAD  FACTOR  95 

load.  Thus  a  generator  of  a  certain  size  and  of  a  certain  manufacture  IB 
given  a  normal  rating  of  100  kva.  This  means  that  the  machine  is  capa- 
ble of  carrying  continuously  a  load  of  100  kva.,  and  that  it  will  also 
carry  an  overload  of  50  per  cent. — 150  kva. — for  1  hr.  after  it  has  been 
continuously  carrying  its  100  kva.  normal  load.  Furthermore,  this  same 
machine  will  carry  135  kva.  continuously  (35  per  cent,  over  its  normal 
rating)  and  hence  can  be  called  a  135-kva.  maximum — or  continuous — 
rating  machine. 

The  generator  discussed  had  a  normal  rating  of  100  kva.  and  a  con- 
tinuous or  maximum  rating  of  135  kva.  The  present  tendency  is  to 
give  all  electrical  machinery  only  one  rating — the  continuous.  This 
will  tend  to  minimize  the  confusion  relating  to  ratings  which  now  exists. 
Practically  all  generators  and  transformers  are  now  rated  only  on  the 
maximum  (the  continuous-carrying-capacity)  basis. 

109.  The  Importance  of  Maintaining  the  Plant  Factor  as 
High  as  Possible  will  be  apparent  from  a  consideration  of  the 
discussion    given    in    Art.  87   relating  to  load  factor.     In 
general,   the  lower  the  plant  factor  of  a  station  the  greater 
will  be  its  cost  of  producing  energy. 

110.  Capacity  Factor,  a  value  sometimes  used,  has  about  the 
same  significance  as  plant  factor.     Capacity  factor  is  not 
mentioned  in  the  A.  I.  E.  E.  STANDARDIZATION  RULES  but  is 
defined  by  G.  I.  Rhodes*  as:  "the  ratio  of  the  average  load  to 
the  rated  capacity  of  the  equipment  supplying  that  load."     It 
might  be  properly  called  output-capacity  factor.     As  with 
plant  factor,  this  value  will  not  have  a  definite  meaning  unless 
the  method  used  in  rating  the  output  capacity  of  the  equipment 
in  question  is  specified.     The  "continuous"  method  of  rating 
(defined  below)  should  always  be  used  where  feasible. 

NOTE. — "Capacity  factor"  is,  probably,  a  better  and  more  general 
term  than  "plant  factor"  because,  strictly  speaking,  the  word  "plant" 
limits  the  use  of  the  value  (plant  factor)  to  the  total  output  of  a  plant  of 
some  sort.  But  "capacity  factor"  may  be  properly  used  as  relating  to 
the  output  of  an  energy  delivering  plant  or  to  the  output  of  any  indi- 
vidual unit  or  group  of  equipment  in  the  plant  or  station.  Thus,  there 
may  be  a  capacity  factor  for  a  station  and  a  capacity  factor  for  any  gen- 
erator or  motor  generator  in  a  station.  It  is  not  unlikely  that,  because 
of  its  more  general  application,  the  term  "capacity  factor"  may  super- 
sede "plant  factor." 

•  STANDARD  HANDBOOK,  1916;  p.  875. 


96  CENTRAL  STATIONS  [ART.  Ill 

111.  The  Distinction  Between  Plant  Factor,  Load  Factor 
and  Capacity  Factor  should  be  clearly  understood  because  the 
terms  are  sometimes,  though  incorrectly,  used  interchangeably. 
The  term  "load  factor"  is  frequently  used  where   "plant 
factor"  is  really  meant.     Some  writers  of  standing  thus  use 
"load  factor"  incorrectly,  but,  since  the  term  is  accurately 
defined  in  the  A.  I.  E.  E.  STANDARDIZATION  RULES,  it  appears 
best  to  adhere  rigidly  to  the  definition  there  given.     "Load 
factor"  is  the  ratio  of  average  power  to  maximum  demand  while 
"plant  factor"  is  the  ratio  of  average  power  to  rated  station 
capacity.     Furthermore,  "load  factor"  may  relate  either  to 
the  energy  delivering  or  energy  receiving  equipment  while 
"plant  factor"  relates  specifically  to  delivering  equipment. 
The  distinction  between  "plant  factor"  and  "capacity  factor" 
is  that  "plant  factor"  relates  specifically  to  the  total  output 
of  an  energy-delivering  station  while  "capacity  factor"  may 
relate  to  the  output  of  any  energy-delivering  station,  machine, 
system  or  equipment.     Note  that  "plant  factor"  is  really 
a  special  restricted  case  of  "capacity  factor." 

112.  Connected-load  Factor  is  the  ratio  of  the  average  power 
input  to  the  connected  load.     It  is  expressed  as  a  percentage 
and  relates  only  to  receiving  equipment.     As  with  load  factor, 
to  render  this  value  specific  the  period  over  which  the  power 
is  averaged  should  be  specified.     Usually  the  average  is  taken 
over  a  year  and  if  no  period  is  mentioned  a  year  is  ordinarily 
implied.     From  the  definition  just  given  it  follows  that: 

(32)  Connected  load  factor  =  ^age  power  input 

connected  load 

(33)  Average  power  input  =  (con.  load  factor}  X  (con.   load) 


(34)  Connected  load  =  avera9e  power  input 

con.  load  factor 

"Connected  load  "  is  defined  in  a  following  paragraph.  The 
average  power  input  and  the  connected  load  must  be  expressed 
in  the  same  units.  If  the  power  input  is  expressed  in  kilo- 
watts, the  connected  load  should  then  also  be  expressed  in 
kilowatts.  If  the  power  input  is  expressed  in  horse-power, 
the  connected  load  should  be  expressed  in  horse-power.  The 


SEC.  5]    PLANT  FACTOR  AND  CONNECTED-LOAD  FACTOR  97 

connected-load  value  used  should  be  based  on  the  output 
capacity  of  the  equipment  involved,  and  not  on  the  input 
capacity. 

113.  Connected-load  Factors  Are  Most  Useful  in  Finding 
the  Probable  Average  Power  Input  or  the  Probable  Annual 
Energy  Consumption  of  an  installation  when  the  connected 
load  and  the  connected-load  factor  applying  to  it  are  known. 
A  distinguishing  feature  of  connected-load  factor  is  that  it 
relates  only  to  energy  consuming  apparatus.     A  comparison 
of  equation  (32)  with  those  of  (14)  and  (29)  will  disclose  the 
distinction  between  this  and  the  other  factors  herein  consid- 
ered. 

114.  To  Insure  That  a  Connected-load  Factor  Has  a  Definite 
Meaning  it  is  necessary  to  specify  the  basis  on  which  the 
connected    load    is    computed.     "Connected    load"    should, 
strictly  speaking  (see  definition  given  in  Art.  116)  always  be 
stated  on  a  continuous-rating  basis.     However,  it  is  not  always 
feasible  to  follow  this  method.     A  lighting  "connected  load" 
is  equal  to  the  sum  of  the  wattages  of  all  of  the  lamps  in  the 
installation.     A  motor  "connected  load"  is  equal  to  the  sum 
of  the  rated  (nameplate)  outputs  of  all  of  the  motors.     Motors 
are  usually  rated  in  horse-power  output;  hence,  it  is  usually 
most  convenient  to  reduce  these  horse-power  values  to  equiva- 
lent kilowatt  values  before  adding  them  together.     Motors 
are  now  ordinarily  rated  on  a  "normal"  output  basis  but  a 
"continuous"    rating   is   now   sometimes   given   to   motors. 
It  is  not  improbable  that,  in  the  future,  all  motors  may  be 
rated  on  a  "continuous"  basis.     Hence,  the  method  employed 
in  rating  the  motors  should,  where  motors  are  involved,  be 
specified  when  a  connected  load  factor  is  stated.     At  this 
time,  when  a  connected  load  factor  is  given  for  a  motor  load 
and  the  method  whereby  the  motors  were  rated  is  not  specified 
it  may  be  assumed  that  "normal"  ratings  are  inferred.     Ex- 
amples which  follow  illustrate  the  method. 

EXAMPLE. — What  is  the  connected-load  factor  of  the  installation  shown 
in  Fig.  54  on  a  1-day  (10-hr.)  basis?     The  watt-hour  meter  records  234 
kw.-hr.  as  having  been  used  during  a  certain  10-hr,  day.     Full  load  out- 
put (nameplate,  normal  basis)  ratings  in  kilowatts  are  indicated  in  the 
7 


98 


CENTRAL  STATIONS 


[ART.  115 


illustration  near  each  piece  of  apparatus  in  the  illustration.  SOLUTION. — 
The  average  load  for  the  10-hr,  day  is:  234  H-  10  =  23.4  kw.  Now 
substitute  in  equation  (32):  Con.  load  factor  =  (average  load)  -r-  (con- 
nected load)  =  23.4  ^  (2.5  +  5.5  +  30  +  7.5  +  13)  =  23.4  +  58.5  =  0.4 
=  40  per  cent.  Hence,  the  connected-load  factor  of  this  installation  is 
40  per  cent. 

EXAMPLE. — The  combined  motor  and  lighting  load  diagrammed  in 
Fig.  64  is  installed  in  a  foundry.  What  average  annual  load  may  this 
installation  be  expected  to  impose  on  the  central  station  and  what  will 


FIG.  64. — Illustrating    an    example    of    the    application    of    connected-load 
factor. 

be  the  probable  annual  (8,760-hr.)  energy  consumption?  SOLUTION.— 
The  annual  approximate  connected-load  factor  is  shown  in  Table  100, 
column  C,  to  be  11  per  cent.  The  equivalent  rated  connected  load  (nor- 
mal ratings  of  motors)  in  kilowatts,  is,  as  shown  by  the  power  output 
symbols  P0:  11.2  +  22.4  +  5.6  +  29.8  +  11.2  +  37.3  +  22.4  +  3.7  + 
1.3  =  144.9  kw.  Now  substitute  in  equation  (33):  Av.  power  input  = 
(con.  load  factor)  X  (con.  load)  =  0.11  X  144.9  =  15.9  kw.  Hence, 
the  average  power  load  imposed  by  this  plant  on  the  central^tation  sys- 
tem would,  probably,  be  about  16  kw.  To  ascertain  the  kilowatt-hour 
energy  consumed  annually,  substitute  in  equation  (19) :  Kw.-hr.  expended 
during  period  =  (av.  power)  X  no.  hr.  in  period)  =  15.9  X  8,760  = 
139,284  kw-hr. 

115.  Connected-load  Factor  Equals  the  Product  of  Demand 
Factor  and  Load  Factor  as  will  be  shown.  By  definition,  see 
equation  (32): 


SEC.  5]  PLANT  FACTOR  AND  CONNECTED-WAD  FACTOR     99 
(35) 


But,  as  shown  in  equation  (15): 

(36)  Average  power  input  =  (load  factor)  X  maximum  de- 
mand). Furthermore,  it  can,  on  the  basis  of  definition 
(Art.  116  and  equation  (9)),  be  shown  that: 

maximum   demand 

(37)  Connected  load  =  —  ,  ,    .    — 

demand  factor 

Substituting  the  expressions  for  average  power  input  of  (36) 
and  for  connected  load  of  (37)  in  equation  (35)  the  result  is: 

(38)  Con  -Id  fac  =  ^loadfactor^  x  (max,  dem.)  X  (dem.fac.'). 

maximum  demand 

The  expression  maximum  demand  appears  in  both  numerator 
and  denominator  of  the  above  equation,  hence  "cancels  out" 
and  the  resulting  working  formula  is: 

(39)  Connected-load  factor  =  (load  factor)  X  (demand  factor). 

116.  Connected  Load  is  defined*  as  "the  combined  con- 
tinuous rating  of  all  the  receiving  apparatus  on  consumers' 
premises,  connected  to  the  system  or  part  of  the  system  under 
consideration."     The  output  ratings  should,  where  feasible, 
be  used  instead  of  the  input  ratings. 

EXAMPLE.  —  The  connected  load  on  the  service  shown  in  Fig.  54  is: 
2.5  kw.  +  30  kw.  +  13  kw.  +  7.5  kw.  +  5.5  kw.  =  58.5  kw. 

117.  A  Graph  for  Quickly  Computing  the  Kilowatt-hour 
Energy   Consumption   of   an   Installation   When   the    Con- 
nected-load Factor  and  the  Connected  Load  Are  Known  is 
given  in  Fig.  65.     The  application  is  explained  in  the  follow- 
ing examples.     This  graph  may  also,  when  the  connected- 
load  factor  is  known,  be  applied  conveniently  for  determining, 
by  inspection,  the  equivalent  hours  used  per  day  of  the  total 
connected  load. 

*  A.  I   E.  E.  STANDARD  IATION  Rou. 


100 


CENTRAL  STATIONS 


[ART  117 


EXAMPLE. — In  a  certain  plant  the  total  rated  capacity  of  all  of  the  motors 
is  60  h.p.  The  connected-load  factor  is  known  to  be  25  per  cent.  What 
will  be  annual  energy  consumption?  SOLTTTION. — The  graph  of  Kg.  65 
indicates  that,  with  a  connected-load  factor  of  25  per  cent.,  the  annual 
energy  consumption  will  be  1,634  kw.-hr.  per  h.p.  installed.  Hence,  the 
annual  energy  consumption  will  be:  60  h.p.  X  1,634  =  98.040  kw.-hr. 


01     23456     7    8    9    10  II    12  13   14  15  J6  17  18  19  20  Zl  22  23  24 
Equivalent  Hours  Use  Per  Day  of  Total  Rated  Capacity  ( 'Connected  Load) 

0       544.6    1089     1634     2178     2723    3267    3812     4357    4901     5446    5990    65JS 

272.3    816.9     1361      1906     2451     2995    3540    4084   4629     5174     57/8    6263 

Kilowatt-Hours,  per  Year,  per  H>  Installed 

FIG.  65. — Graph  showing  relation  of  connected-load  factor  to  equivalent 
hours  use  of  connected  load. 

It  will  also  be  noted  from  Fig.  65  that  a  connected-load  factor  of  25  per 
cent,  is  equivalent  to  a  6-hr,  use  per  day  of  the  total  capacity  or  connected 
load  installed. 

EXAMPLE.— If  the  connected-load  factor  of  an  installation  is  40  per 
cent.,  it  means  that  the  energy  consumed  by  this  installation  is  the  same 
as  that  which  would  be  consumed  by  all  of  the  connected  load  if  it  were 
operated  at  rated  (nameplate)  output  for  9>£  hr.  per  day  (see  Fig.  65) 
every  one  of  the  365  days  of  a  year 


SECTION  6 


LOAD  GRAPHS  AND  THEIR  SIGNIFICANCE 

118.  A  Load  Graph,  or  as  it  is  sometimes  called,  a  load  curve, 
is  merely  a  graphic  record  of  the  power  loads  which  have  been 
imposed  on  a  station  or  on  some  electrical  unit  at  all  of  the 
different  instants  during  a  certain  period  of  time.  The  illus- 
trations (Figs.  66  to  87,  which  are  based  largely  on  data  pro- 


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FIG.  66. — Load  curve  for  New  York  Central  Station  service  on  the  maxii 
output  day  of  the  year  1915. 

posed  by  G.  I.  Rhodes*)  show  examples  of  load  graphs.  These 
graphs  are  usually  plotted  with  the  power  values  vertically, 
that  is,  along  the  ordinates  of  the  graphs.  Time  is  plotted 
horizontally,  that  is,  along  the  abscissae  of  the  graphs. 

NOTE. — The  area  included  within  the  load  graph — indicated  by  the 
shaded  portions  in  the  illustrations — represent  energy.  That  is,  the 
product  of:  Power  X  time.  Thus,  in  Fig.  66  the  shaded  portion  of  the 
graph  is  proportional  to: 

Average  poic*r  X  hours  =  122,356  kw.  X  24  hr.  =  2,937,538  kw.-hr 

*  STANDARD  HANDBOOK. 

101 


102 


CENTRAL  STATIONS 


[ART.  119 


119.  There  are  Two  General  Methods  Whereby  the  Data 
for  Plotting  Load  Graphs  May  be  Obtained. — Method  1.— 
Where  graphic  instruments  are  installed  the  load  imposed  at 
any  instant  can  be  readily  ascertained  from  the  record  made 
by  the  instrument.  In  fact,  the  record  strip  of  a  graphic 
wattmeter  is  its  load  graph.  Method  2. — Where  graphic  in- 
struments are  not  installed,  the  power  values  for  plotting  the 
graph  may  be  read  at  equidistant  intervals  from  indicating 
instruments  connected  into  the  circuit  under  consideration. 
The  frequency  with  which  the  readings  should  be  taken  will 
be  determined  to  a  large  extent  by  the  character  of  the  load 
under  consideration.  If  the  load  is  subject  to  wide  and  con- 

3500 

3000 


Energy  Required  for 

CHarging  ofJOO     \ 


PM. 


FIG.  67. — Showing  effect  on  a  typical  central  station  load  graph  of  adding 
the  charging  load  of  a  hundred  commercial  electric  vehicles.  (Station  is 
steam  driven  and  in  a  city  of  100,000  inhabitants.  Graph  is  for  Feb. 
1,  1916.) 

tinual  fluxation,  it  may  be  desirable  to  take  readings  every 
minute  or  even  every  half  minute,  but  if  the  load  is  reasonably 
steady — that  is,  changes  in  value  slowly — it  will  usually  be 
sufficient  to  take  a  reading  at  the  end  of  every  15-min.  time 
interval.  Where  an  indicating  wattmeter  is  available,  the 
power  values  thus  obtained  from  it  may  be  plotted  against 
time  into  the  graph.  If  no  wattmeter  is  available  but  an 
ammeter  or  a  voltmeter  is,  then  both  instruments  should  be 
read  simultaneously  at  the  end  of  each  time  interval.  Then 
the  product  of  the  current  and  voltage  thus  obtained  will,  on 
direct-current  circuits,  be  the  watts-power  expenditure  at  the 
specified  instant. 


SKC.  6]      LOAD  GRAPHS  AND  THEIR  SIGNIFICANCE 


103 


NOTE. — On  alternating-current  circuits  unless  the  power  factor  hap- 
pens to  be  100  per  cent.,  which  is  not  likely  to  be  the  case,  the  product 
of  the  volts  and  amperes  will  not  represent  watts;  hence,  with  alternating- 
current  circuits,  it  is  inconvenient  to  obtain  the  power-expenditure 
values  unless  an  alternating-current  wattmeter  is  used. 

EXAMPLE. — The  graph  C  of  Fig.  67  was  plotted  from  the  following 
values : 


Kilowatts  power 


Kilowatts   power 


12:00  (Noon) 

1,000 

3:15                      2,380 

12:15 

850 

3:30 

2,360 

12:30 

1,050 

3:45                      2,350 

12:45 

1,250 

4:00 

2,340 

1:00 

1,600 

4:15 

2,450 

1:15 

1,800 

4:30 

2,520 

1:30 

1,950 

4:45 

2,600 

1:45 

2,050 

5:00 

2,700 

2:00 

2,250 

5:15 

2,800 

2:15 

2,300 

5:30                      2,900 

2:30 

2,325 

5:45                       2,820 

2:45 

2,350 

6:00                      2,630 

8:00 

2,400 

etc.                         etc. 

120.  The  Importance  of  a  Thorough  Appreciation  of  the 
Significance  of  Load  Graphs  should  be  understood.  A  load 
graph  indicates  at  a  glance  the  general  character  of  the  load 
which  is  being  imposed  and  brings  out  the  facts  much  more 
forcefully  than  will  a  couple  of  columns  of  figures.  The  higher 
the  load  factor  (ratio  of  average  load  to  maximum  demand) 
the  lower,  in  general,  the  cost  of  energy  production  will  be. 
Obviously,  the  more  nearly  the  graph  of  a  load  approximates 
a  horizontal  line  the  nearer  will  the  conditions  be  to  the  ideal. 
That  is,  to  economize  energy  production  the  "valleys"  of  a 
load  graph  should  be  filled  in  and  the  "peaks "  should  be  lopped 
off.  An  inspection  of  a  load  graph  will  indicate  just  at  what 
hours  of  the  day  the  "valleys"  and  the  "peaks"  occurred  and, 
with  this  information  available,  suitable  measures  may  be 
taken  to  "even  up  the  graph." 

NOTE. — For  these  reasons  every  generating  station  should  keep  load 
graphs  as  an  important  feature  of  the  station  records.  The  graphs  can- 


104 


CENTRAL  STATIONS 


[ART.  121 


where  a  graphic  wattmeter  is  not  installed,  be  plotted  daily  from  the 
station  log. 

121.  The  Unit  for  the  Ordinate  Values  of  a  Load  Graph  is 

ordinarily  a  kilowatt.  In  certain  instances  it  may,  where 
constant-potential  circuits  are  involved,  be  desirable  to  use  the 
ampere  as  the  ordinate  unit,  because  then  the  ampere  values 
can,  on  direct-current  circuits,  be  translated  into  watts  or 
kilowatts  by  multiplying  by  the  constant-potential  voltage. 

122.  The  Period  Which  Should  Be  Comprehended  by  a 
Load  Graph  is  a  thing  which  local  conditions  must  decide. 
Where  plants  operate  24  hr.  a  day  it  is  usual  to  plot  the  graphs 
relating  thereto  so  that  their  horizontal  lengths  represent  a 


y]\ 

J 

^       m 

A 

J 

o 

fKxv 

j' 

V 

>rn 

\j\ 

\ 

rtf 

7 

1 

ill! 

f""!i 

M 

\ 

o  

J 

1 

111  III 

\ 

FIG.  68. — Annual  or  yearly  load  graph. 

24-hr,  period.  When  a  plant  operates  only  8  or  12  hr.  daily 
then  it  is  sufficient  if  the  length  of  the  graphs  represents  only 
the  8-  or  12-hr,  period.  Most  load  graphs  are  plotted  on  a 
24-hr,  basis  as  an  examination  of  the  accompanying  illustra- 
tions will  verify.  Frequently  it  is  desirable  to  plot  graphs  on 
a  yearly  basis  as  illustrated  in  Fig.  68  that  one  may  study  the 
distribution  of  the  energy  expenditure  over  the  entire  year. 

123.  Loads  of  Different  Types  Have  Their  Typical  Load 
Graphs. — That  is,  the  graph  for  any  electric-lighting  load  will 
follow  the  general  contour  of  that  shown  in  Fig.  69.  Indus- 
trial or  factory  loads  will  all  have  graphs  of  the  general  outline 
suggested  in  Fig.  70.  This'same  condition  holds,  in  a  broad 
way,  for  all  of  the  different  loads  of  different  classification 


SEC.  6]      WAD  GRAPHS  AND  THEIR  SIGNIFICANCE 


105 


which  a  station  may  be  called  upon  to  serve.  It  is  for  this 
reason  that  the  discussion  of  the  load  graphs  of  the  different 
types  which  follows  is  given.  These  are,  for  the  most  part, 


120 


IW 


FIG.  69. — Typical  24-hour  load  graph  for  an  electric  lighting  load. 

based  on  power-plant  economics  data  developed  by  George  I. 
Rhodes. 

124.  The  Load  Graph  of  a  Typical  Electric-lighting  Load  in 
a  town  or  city  is  shown  in  Fig.  69.     Where  there  is  little  or  no 


IdU 

too 

Industr 

ilo-  Factory  Load 

WeeklyUx. 
Annva/loc 

aSEr- 

v  'Factor* 

5<J^/'N 

T 

46*f           "  N 

Kilowatts 

0  A  O  C 
b  C.  0  C 

>, 

I- 

x,. 

" 

$ 
FIG.  70. 

L 

Hfl   Z   J  4  S  6   7   8  9  10  II  li 
A.M. 

—  Typical  24-hour    oad  graph 

Wl  2    3456793  10  IHOJ2 
PM 

for  an  industrial   or  factory  load. 

demand  for  energy  for  motors  or  railways,  the  power  con- 
sumption will  vary  over  the  24  hr.  approximately  as  dia- 
grammed in  the  illustration.  The  full-line  graph  indicates  a 


106  CENTRAL  STATIONS  [ART.  125 

typical  winter-day  power  demand  while  that  which  is  dotted 
shows  the  demand  on  a  typical  summer  day.  The  maximum 
demand  occurs  in  the  winter  time  between  4  and  6  o'clock  in 
the  evening  because  at  this  time  most  of  the  stores  and  offices 
and  many  of  the  residences  are  using  a  maximum  of  light.  A 
maximum  "peak"  for  the  year  usually  occurs  in  December 
(Fig.  66).  In  the  summer  the  lighting  "peak"  is  imposed  in 
the  evening — about  8:00  P.M. — and  it  is  of  considerable  lower 
value  than  the  winter  peak. 

NOTE. — The  minimum  demand  for  lighting  energy  occurs  between 
9  o'clock  in  the  morning  and  2  o'clock  in  the  afternoon  in  winter  and  be- 
tween 4  o'clock  in  the  morning  and  5  o'clock  in  the  afternoon  in  summer. 
A  noticeable  characteristic  of  lighting  loads  is  the  abruptness  with  which 
the  energy  consumption  increases  (from  A  to  B,  Fig.  69)  in  the  evening 
and  also  the  suddenness  with  which  it  decreases  (from  B  to  C,  Fig.  69). 
after  the  shops  and  offices  close  in  the  evening.  The  load  factors  for 
typical  electric-lighting  loads  of  the  general  characteristics  indicated  in 
the  graph  of  Fig.  69  are  noted  in  the  illustration.  As  there  suggested, 
the  annual  load  factor  is  about  23  per  cent. 

125.  A  Typical  Graph  for  an  Industrial  Load  is  represented 
in  Fig.  70.     The  load  is  a  minimum  during  the  hours  when  the 
industrial  plant  is  not  in  operation.     But  the  demand  increases 
very  abruptly  at  about  6  o'clock  in  the  morning  and  attains 
the  maximum  for  the  day  about  7: 00  or  8:00  A.M.     At  the 
noon  hour  the  graph  drops  almost  vertically  downward  and 
rises  again  at  1  o'clock  when  the  machines  and  lining  equip- 
ment is  again  cut  into  service.     It  should  be  noted  that  the 
afternoon  peak  occurs  shortly  after  1  o'clock,  but  it  is  seldom 
as  great  as  the  morning  peak.     The  power  demands  imposed 
by  an  industrial  plant  are  about  the  same  in  winter  as  in  sum- 
mer.    The  annual  load  factor  will  be  about  46  per  cent. 

126.  A  Typical  Load  Graph  for  a  City  Street  Railway  is 
delineated  in  Fig.  71.     There  are  two  pronounced  peaks  occur- 
ring at  about  8:00  A.M.  and  6:00  P.M.     These  are  due,  re- 
spectively, to  the  demands  imposed  by  the  transportation  of 
employees  to  and  from  work.     The  minimum  demand  occurs 
about  3  o'clock  in  the  morning.     A  graph  for  a  typical  sum- 
mer day  has  the  same  general  contour  as  that  for  a  winter  day. 
But  the  summer-day  demands  are,  at  every  hour  of  the  24, 


SEC.  6]      LOAD  GRAPHS  AND  THEIR  SIGNIFICANCE 


107 


less  than  those  in  winter.  The  principal  reason  for  this  con- 
dition is  that  the  electric  heaters  on  the  cars  require  consider- 
able energy  in  the  winter.  In  some  cities  it  may  occur  that 
the  summer  traffic  is  heavier  than  the  winter,  but  it  is  seldom 


°I2NT/    ?   J   4    5   6    7   8  9  10  Jl  IZNfil   23   4   5    67   8  9  10 IINII2 

A.M.  PM 

FIG.  71. — Typical  24-hour  load  graph  for  a  city  street  railway  load. 

that  the  summer  peak  is  higher  than  the  winter  peak.  The 
annual  load  factor  of  a  load  of  the  general  characteristics  indi- 
cated in  Fig.  71  is  about  35  per  cent. 


Inter J>rbbn  (Street  R 
Winter 

Sumf>err59* 
I  \Ave^age*60°l' 
Week/A  [W<n(er\=60_7. 

<YtHte 


lway 


lira  I   23456789  10  II  IZMN.I  2    3456789  10  IINTR 

A.M.  R  M. 

FIG.  72. — Typical  24-hour  load  graph  for  an  interurban  railway  load. 

127.  Interurban  Street  Railways  usually  show  a  graph  about 
like  that  outlined  in  Fig.  72.  The  peak  occurs  at  about  7 
o'clock  in  the  evening  and  is  caused  by  the  heavy  traffic 


108 


CENTRAL  STATIONS 


[ART.  128 


due  to  people  riding  home  from  the  towns  where  they  have 
been  employed  or  visiting.  The  annual  load  factor  for  a  load 
of  this  character  is  about  47  per  cent. 


A.M. 


6789/0  1INM 
RM. 


FIG.  73. — Typical  24-hour  load  graph  for  a  combined  electric  lighting  and 
industrial  load. 

128.  By  Combining  a  Lighting  and  an  Industrial  Load  on 
the  same  energy  supply  source  the  resultant  load  imposed  on 
the  generating  equipment  will  be  of  the  character  indicated 


456 

P.M. 

FIG.  74. — Typical   24-hour  load  graph  for  a   combined   lighting   and  city 
railway  load. 

in  Fig.  73.  Note  that  due  to  the  diversity  of  the  demands 
between  the  loads  of  these  two  different  types  there  is  a 
tendency  toward  "smoothing  out  the  hollows"  in  the  load 
curves  and  that  the  annual  load  factor  for  such  a  combined 


SEC.  6]     LOAD  GRAPHS  AND  THEIR  SIGNIFICANCE          109 

load  is  about  40  per  cent,  as  against  23  per  cent,  for  the  un- 
combined  lighting  load  of  Fig.  69.  Therefore,  it  follows  that 
a  material  economy  in  energy  production  results  where  loads 
of  Dissimilar  characteristics  can  thus  be  consolidated. 

129.  A  Combined  Lighting  and  Street  Railway  Load  will 
produce  a  graph  of  the  general  contour  suggested  in  Fig.  74. 
The  annual  load  factor  resulting  is  only  32  per  cent,  as  against 
40  per  cent,  for  a  combined  lighting  and  industrial  load  (Fig. 
73).  This  is  largely  due  to  the  fact  that  an  industrial  load  is 
about  the  same  in  the  summer  as  in  winter,  whereas,  both 
lighting  and  railway  loads  are  considerably  greater  in  the 
winter  than  in  the  summer. 


FIG.  75.— Typical   24-hour  load  graph  for  a   combined  electric  lighting. 
industrial  and  interurban  railway  load. 

130.  When  Lighting,  Industrial  and  Interurban  Railway 
Loads  are  combined  the  resulting  24-hr,  load  graph  will  follow 
about  the  contour  plotted  in  Fig.  75.    The  annual  load  figure 
is  then,  approximately,  42  per  cent. 

131.  A   Combination  of  Lighting,  Industrial,  Interurban 
and  Street  Railway  Loads  will  impose  on  the  supplying  equip- 
ment demands  which  will  vary  through  the  24  hr.  of  the  day 
somewhat  as  outlined  in  Fig.  76.    The  annual  load  factor  will 
probably  be  in  the  neighborhood  of  45  per  cent. 

132.  Load  Graphs  for  Large  Cities  are  shown  in  Figs.  66 
and  77.    These  have  been  plotted  respectively  for  New  York 
and  Chicago  from  actual  operating  data.    Both  indicate  the 


110 


CENTRAL  STATIONS 


.  133 


results  that  may  be  expected  bj  combining  loads  of  different 
characteristics  on  one  supply  system.  The  graph  of  Fig.  66 
comprehends  loads  handled  by  two  companies  which  operate 


IK 


20 


Cbmbmed  Lighting,  Industrial,  Interurbanland  City-Street-Railway  Load 

1  -k!  ' 


pf^lAi&p-OTl 

inuaf Load  factor  •  45° 


KHII   23456733  W  II  KNJil  23456783  10  lltfM 
A.M.  P.M. 

FIG.  76. — Typical  24-hour  load  graph  for  a  combined  lighting,  industrial, 
interurban  and  city  railway  load. 

in  New  York — The  New  York  Edison  Company  and  the 
United  Electric  Light  and  Power  Company.  The  load  factor 
is  approximately  50  per  cent.  Fig.  77  shows  total  load  for  a 


300,000 
^250.000 
\200flOO 
^150,000 
100,000 
SOflOO 

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ntt 

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a/No* 

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J 

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17  \s 

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i 

ag 

H: 

0 

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I 

1     | 

1  1  ii 

II 

FIG.  77. — Load  graphs  for  Chicago,  FIG.    78. — Typical    graph    of   the 

111.,  on  typical  summer  and  winter       loads  on  an   office-building   isolated 


day      in     1915. 
Edison  Co.) 


(Commonwealth       plant. 


typical  summer  and  winter  day  which  the  Commonwealth 
Edison  Company  of  Chicago  serves. 

133.  A  Load  Imposed  on  an  Office  Building  Isolated  Plant 
will  have  the  general  characteristics  graphed  in  Fig.  78.     The 


SEC.  li]      LOAD  GRAPHS  AND  THEIR  SIGNIFICANCE 


111 


peak  at  8:00  A.M.  is  due,  for  the  most  part,  to  the  energy 
taken  by  the  electric  elevators.  The  evening  peak  between 
4  and  6  o'clock  is  due  largely  to  the  power  required  for  light 
but  the  elevator  power  also  has  its  effect  at  this  tune. 

134.  A  Hotel  Isolated  Plant  will  usually  have  imposed  on  it 
a  load  which  will  vary  with  the  time  somewhat  as  outlined  in 
Fig.  79.     The  peak,  which  occurs  in  the  evening,  is  due,  for 
the  most  part,  to  electric  lighting,  but  the  electric  elevator 
requirements  usually  add  an  appreciable  share  to  the  demand 
at  this  time. 

135.  A  Department  Store  Isolated  Plant  will  have  imposed 
on  it  a  load  of  the  typical  properties  graphed  in  Fig.  80.     The 


T\ 

1 

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Kh 

Wmt 

erDay—.>l 

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Winfer^Day-    - 

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nmer    X<T 

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Day 

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4  6  8  10  12 

FIG.  79. — Typical  graphs  of  the  load 
on  a  hotel  isolated  plant. 


FIG.  80. — Typical  graphs  of  the 
loads  on  a  department-store  isolated 
plant. 


peak  occurs  quite  early  in  the  evening,  just  about  the  time 
the  store  closes  and  the  demand  drops  off  abruptly  after 
closing  time.  A  somewhat  unusual  feature  which  character- 
istic of  department-store  loads  is  that  at  certain  hours  of  the 
day  the  summer  off-peak  load  may  be  greater  than  the  load 
imposed  at  the  same  hours  in  the  winter  time. 

136.  How  the  Addition  of  Off-peak  Loads  Will  Improve 
the  Load  Factor  is  brought  out  by  the  graph  of  Fig.  67. 
The  area  A  represents  the  energy  required  by  the  regular 
load  imposed  on  the  system  or  plant  in  question  which  operates 
in  a  city  of  approximately  100,000  inhabitants.  The  graph 
shows  the  conditions  on  Feb.  1,  1916.  The  load  factor  for 
load  A  is  about  59.6  per  cent.  Now,  if  the  energy,  B,  required 


112 


CENTRAL  STATION'S 


[ART.  137 


for  the  charging  of  100  commercial  electric  vehicles  be  added 
during  the  "valley"  the  load  factor  is  increased  to  63.2  per 
cent,  and  a  consequent  reduction  in  energy-production  cost 
will  result. 


?500 

3000 

£2500 

§2000 

.5 1500 

^1000 

500 


3500\ 


12  2  4  6  8  10  12  2  4  6  \8  tO  12 

r< AM >rt. -P.M.- H 

I -September  to  February 


K 


2  4   6  8  10  12  2  4   6  8  10  12 
k- A.M.- ->X- P.M.- >« 

II"  March  to  August 


FIG.  81. — A   comparison   of  winter  and   summer  load   graphs.     The   1912 
graphs  were  plotted  from  observed  data.     1916  graphs  were  estimated. 

137.  A  Comparison  of  Winter  and  Summer  Load  Graphs 
for  the  load  imposed  on  the  station  in  a  city  of  approximately 
180,000  inhabitants  is  given  graphically  in  Fig.  81.  This 
illustration  also  shows  how,  in  the  particular  case  under  con- 


rzo 

(10 
100 

sc 

70 

\ 

J 
I 

.  

*f 

1 

/ 

-_ 

\ 

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s 

'j 

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1 

S 

i 

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i 

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re 

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i 

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\ 

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20 

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1 

FIG.  82. — 24-hour  load  graphs  for  three  small  towns  in  Georgia.     (W.  Rawson 
Collier,  Electrical  Review,  Nov.  13,  1915.) 

sideration,  the  load  was  increased  during  the  period  of  4  years 
(from  1912  to  1916). 

138.  The  Characteristic  Load  Graphs  for  Small  Towns  may 
vary  considerably  as  evidenced  by  the  data  of  Fig.  82,  wherein 


SEC.  6]      LOAD  GRAPHS  AND  THEIR  SIGNIFICANCE 


113 


A,  B  and  C  are  the  curves  for  three  different  municipalities. 
This  condition  does  not  hold  in  the  case  of  the  larger  cities, 
because  with  them  the  load  graphs  for  all  usually  exhibit 
similar  characteristics.  All  three  of  the  towns  (Fig.  82)  have 
approximately  4,800  inhabitants.  In  all  three  the  water- 
works pumps  are  electrically  driven.  In  town  A  during  the 
dry  season  the  motors  driving  the  compressors  for  the  air- 
lift wells  operate  during  the  night  and  also  for  several  hours 
in  the  morning.  That  is,  they  operate  from  9:00  P.M.  to 
6:00  A.M.  There  is  a  fair  consumption  of  energy  for  the 
residential  commercial  lighting  and  there  is  a  reasonably  good 
street  lighting  system. 


April  19, 1915 


_2   4    6   8  10  12  2   4    6   8  K)  I? 

•AM. "& PM. -4 

[KwHrs-l  143 
\LoadFactor  -19?  *h 

•1703 
"*-    \Lood Factor -2&4* 

FIG.  83. — Load  graphs  for  a  small 
city  central  station.  (Those  shown 
are  for  McPherson,  Kansas,  4200 
population.) 


1/000 

Ik  I  500 
*/000 


FIG.  84. — Another  form  of  annual 
load  graph.  This  indicates  the 
monthly  load  peaks  during  the  year. 


The  requirements  of  the  town  B  are,  in  general,  similar  to 
those  of  A  but  in  B  the  pneumatic  lift  water  motors  are  oper- 
ated from  9:00  A.M.  until  about  2:00  P.M.  In  town  C  the 
water-works  pumps  are  operated  between  midnight  and  4:00 
A.M.  In  instances  such  as  that  just  described  it  might  be 
possible  to  arrange  with  three  different  towns  which  are  all 
fed  from  the  same  electricity  supply  system  to  operate  their 
water-works  motors  at  such  times  that  the  demands  imposed 
by  them  would  not  coincide.  Where  this  can  be  arranged 
the  load  factor  of  the  combined  load  thus  imposed  may  be 
materially  increased.  A  24-hr,  load  graph  for  another  small 
city  of  4,200  population  (1910  census),  is  given  in  Fig.  83. 
In  this  town  the  morning  peak  is  not  as  pronounced  as  it  is, 


114 


CENTRAL  STATIONS 


[ART.  139 


for  example,  in  the  graph  of  Figs.  66  and  77,  but  there  is  no 
decided  "valley"  at  the  noon  hour  period.  This  indicates  the 
lack  of  an  industrial  load,  which,  if  added  to  this  system  would, 
probably,  materially  increase  the  load  factor. 


WNighS 


1913 


Apr// 


July 


Oct 


6AM  l2(Noorj  6PM.  IZfNight) 

FIG.  85. — A  "contour-map"  central  station  load  graph. 

139.  Annual  Load  Graphs  may  be  plotted  as  suggested  in 
Figs.  68  and  84  which  will  indicate  how  the  consumption  varies 
over  the  entire  year.  Frequently,  such  graphs  are  plotted 
as  in  Fig.  84,  which  indicate  the  maximum  load  peak  for  each 


SEC.  6]      LOAD  GRAPHS  AND  THEIR  SIGNIFICANCE 


115 


month  rather  than  the  variation  of  power  demand  throughout 
the  period  comprehended  by  the  graph. 

140.  A  "Contour -map"  Load  Graph  is  shown  in  Fig.  85.* 
In  load  graphs  rendered  in  accordance  with  the  usual  method 
(see  the  preceding  illustration)  the  values  are  ordinarily  plotted 
between  power  in  kilowatts  and  the  hours  of  the  day — that  is 
to  "two  dimensions."  In  the  graph  of  Fig.  85,  a  third  set 
of  values — the  days  in  a  year  or  a  series  of  years — is  intro- 


saoo. 


45flOQ 
40000 


35,000 


30,000 


l$000 


JQOOO 


SOOO 


PlanfBL 


456    7  8   9   10  II  I2\l   2   3  4   5   6    7   8   9  /O  II  h 
M  N  M 

FIG,  86. — Illustrating  method  of  adding  two  load  graphs  to  obtain  a  resultant 
or  total  load  graph. 

duced.  Thus  the  graph  is  plotted  to  "three  dimensions." 
The  result  is,  instead  of  a  load  graph  in  one  plane,  a  series  of 
load  graphs  in  space.  When  drawn  on  paper,  the  diagram  is 
similar  in  appearance  to  a  typographical  relief  map  of  a  hilly 
locality — or  to  a  weather  map  which  shows  the  variation  of 
the  barometric  pressure  over  a  given  area.  The  values  plotted 
in  Fig.  85,  indicate  the  kilowatt  loads — and  the  time  of  the 
day  and  the  time  of  the  year  of  their  occurrence — on  the  system 
of  the  Company,  de  Mide  de  La  France.  The  values  near 

•  Max  DuBoU  in  La  Lumiert  Electric,  May  8,  1915. 


11G 


CENTRAL  STATIONS 


[ART.  141 


the  power  contour  lines  in  the  graph,  which  in  a  typographical 
map  would  indicate  elevations  in  feet,  indicate  the  power 
loads  in  kilowatts. 

NOTE  that  the  total  load  on  the  system  has  increased  materially  from 
April,  1913,  to  January,  1915.  The  beginning  of  the  great  war  early  in 
1914  caused  a  sudden  decrease  in  the  demand  for  energy,  which  is  clearly 
indicated  by  the  shading  of,  and  the  contour  lines  on  the  map.  A  load 
graph  like  this,  which  is  plotted  in  three  dimensions,  will  forcibly  indi- 


IIOG 


10CO 


4        6 

k— AM,- 

FIG.  87. — Showing  how  a  storage  battery  may  be  charged  "off  peak" 
and  discharged  on  the  "peak  "  thereby  increasing  the  load  factor  of  the 
load  imposed  on  generating  equipment. 

cate  not  only  at  which  times  of  the  day,  but  also  at  what  times  of  the  year 
the  energy  consumption  of  the  system  is  small.  With  this  information 
available,  suitable  efforts  may  be  made  to  obtain  loads  which  will  "fill 
in"  and  "valleys"  in  the  map — not  only  the  daily  "valleys"  but  also 
the  monthly  and  yearly  "valleys" — and  thereby  increase  the  overall 
load  factor. 

141.  The  Method  of  Adding  to  Load  Graphs  to  obtain  their 
resultant  is  shown  graphically  in  Fig.  86.  Wherein,  graphs 
A  and  B  are  added  together  giving  resultant  graph  C.  Thus, 


SEC.  6]      LOAD  GRAPHS  AND  THEIR  SIGNIFICANCE  117 

the  height  of  any  point  in  the  total  graph,  C,  above  the 
horizontal  axis  is  equal  to  the  sum  of  the  distances  of  each  of 
the  two  component  graphs  above  the  horizontal  axis  meas- 
ured along  the  ordinate  under  consideration.  Thus,  distance 
DE  =  EF  +  EG. 

142.  How  a  Storage  Battery  May  Be  Used  for  Modifying 
the  Load  Demands  Imposed  on  Generating  Equipment  is 
illustrated  by  the  load  graphs  of  Fig.  87.  With  no  storage 
battery  the  load  imposed  on  the  generating  equipment  is 
indicated  by  the  graph  JKBCDEFGI  and  the  load  factor  is 
then  rather  low.  But  with  a  storage  battery  arranged  for 
"off-peak"  charging  and  "on-peak"  discharging,  the  load 
imposed  on  the  generating  equipment  would  be  represented 
by  the  graph  ABCDFGH  and  the  load  factor  would  be 
relatively  high. 


SECTION  7 
GENERAL  PRINCIPLES  OF  CIRCUIT  DESIGN 

143.  In  the  Actual  Design  of  Circuits  it  is  not  practicable, 
though  it  may  be  entirely  possible,  to  apply  Ohm's  law  un- 
modified to  an  extensive  circuit.     It  is,  on  the  contrary,  the 
usual  practice  to  consider  only  the  voltage  at  the  generator 
or  at  some  assumed  source  of  energy  and  the  current  in  each 
portion  of  the  circuit  under  consideration.     From  these  values 
the  volts  drop  or  loss  of  potential  in  the  circuit  can  be  readily 
computed  by  methods  to  be  described. 

NOTE. — It  follows  from  Ohm's  law*  that  the  volts  drop  in  any  portion 
of  the  circuit  will  equal  the  product  of  the  resistance  of  that  portion  times 
the  current  of  that  portion. 

144.  The  Features  Which  Should  Determine  the  Sizes  of 
Wires  for  the  Distribution  of  Electric  Energy  are  three,  thus: 
The  wire  selected  should  be  of  such  size:  (1)  that  it  will  convey 
the  electrical  energy  to  the  location  where  it  is  to  be  utilized  without 
an  excessive  drop  or  loss  in  potential;  that  is,  without  excessive 
I  X  R  drop;  (2)  that  the  current  will  not  heat  it  to  a  temperature 
which  will  injure  the  insulation  of  the  wire  or  originate  a  fire;  (3) 
that  the  cost,  due  to  the  power  loss  (the  I2  X  R  loss]  in  the  wire, 
caused  by  the  current  being  forced  through  the  resistance,  will  not 
be  excessive. 

A  conductor  may  satisfy  one  of  these  three  conditions  and 
not  satisfy  the  other  two,  hence,  as  a  general  proposition,  it  is 
always  desirable  to  examine  the  conductor  size  selected  for 
any  given  condition  from  the  three  different  standpoints  out- 
lined above. 

145.  The  Voltage  Drop  Will  Be  Excessive  if  the  Wire 
Selected  Is  Too  Small. — It  may  readily  be  shown*  that  when 
a  current  of  electricity  flows  in  a  conductor  there  is  always  a 
drop  or  loss  in  pressure  or  voltage.     Practically  all  electrical 

*  See  the  author's  PRACTICAL  ELECTRICITY. 
118 


SBC.  7]     GENERAL  PRINCIPLES  OF  CIRCUIT  DESIGN          119 

apparatus  requires  a  certain  minimum  voltage  for  satisfactory 
operation.  With  incandescent  lamp  circuits  it  is  frequently 
imperative  that  the  voltage  drop  be  not  excessive.  The 
reason  is  that  a  small  decrease  in  the  voltage  impressed  on  an 
incandescent  lamp  causes  a  great  decrease  in  the  light  emitted* 
and  a  small  increase  in  voltage  causes  a  great  decrease  in  the 
life  of  the  lamp. 

NOTE. — Hence,  the  voltage  regulation  of  circuits  feeding  incandescent 
lamps  should  be  very  "close."  That  is,  the  allowable  voltage  drop  in 
incandescent-lamp  lighting  circuits  is  small.  With  circuits  supplying 
motors,  a  greater  drop  in  voltage  can  be  allowed  than  on  circuits  supply- 
ing only  lamps.  However,  if  the  voltage  impressed  across  the  terminal 
of  a  motor  is  very  much  lower  than  that  for  which  its  manufacturer  de- 
signed the  motor,  the  motor  will  become  excessively  hot  in  operation  and 
may  blow  its  fuses  or  trip  its  circuit-breaker. 

£  Ohm-*  I  Amp.--** 


\       •< 1  Volt  Drop >l  '--109  Volts 

•  Generator*  1- Current  =  1  Ampere 


•10  Volts  Drop -- — 

I -Current  =  10  Amp. 

FIG.  88. — Voltage  drop  in  conductors. 

146.  The  Principle  of  Voltage  Drop,  sometimes  called  loss  of 
voltage  or  drop  in  potential,  is  best  illustrated  by  the  considera- 
tion of  the  specific  example  of  Fig.  88.  The  generator  serving 
the  circuit  is  supposed  to  maintain  a  pressure  of  110  volts  at 
its  line  terminals.  The  circuit  of  Fig.  88  has  a  total  resistance 
of  1  ohm — %  ohm  to  each  side  circuit.  Assume  that  each 
incandescent  lamp  connected  across  the  circuit  will  permit  a 
current  of  %  amp.  to  flow.  With  only  two  lamps  connected 
to  the  circuit,  as  shown  at  /,  the  current  in  the  circuit  will  then 
be  1  amp.  Also  the  voltage  drop  in  the  circuit  will  (by  Ohm's 
law)  be:  1  amp.  X  1  ohm  =  1  volt.  Then  the  voltage  at  the 
lamps  would  be  the  voltage  impressed  on  the  circuit  minus 
the  drop.  That  is,  the  voltage  at  the  lamps  would  be:  110 
volts  -  1  volt  =  109  volts. 

Now  with  10  lamps  burning,  as  at  //,  the  drop  would  then 

•  See  the  author's  AMERICAN  ELECTRICIANS'  HANDBOOK. 


120  CENTRAL  STATIONS  [ART.  147 

be:  10  amp.  X  1  ohm  =  10  volts.  Then  the  voltage  impressed 
across  the  lamps  would  be:  110  volts  —  10  volts  =  100  volts. 
Obviously,  with  the  condition  of  7,  the  two  lamps  would  have 
109  volts  impressed  across  them,  whereas  with  the  condition 
of  II  they  would  have  100  volts  impressed  upon  them. 

147.  Incandescent  Lamps  Cannot  be  Manufactured  Which 
Will  Operate  Satisfactorily  Over  a  Wide  Range  of  Voltage. — 
Hence,  the  solution  of  a  condition  such  as  that  described  in 
the  above  example  would  be  to  use  110-volt  incandescent 
lamps  and  so  proportion  the  conductor,  that  with  all  of  the 
lamps  on  the  circuit  burning  simultaneously,  there  would  not 
be  more  than  a  volt  or  two  total  drop  in  the  entire  circuit, 
instead  of  10  volts  drop  as  shown  at  Fig.  88,77.     How  con- 
ductors may  be  proportioned  to  thus  maintain  the  drop  at  a 
minimum  will  be  described  in  the  material  which  follows. 

148.  A  Large  Voltage  Drop  in  a  Conductor  Also  Indicates 
a  Large  Power  Loss  in  That  Conductor. — It  is  often,  from  a 
standpoint  of  economics,  advisable  to  use  a  conductor  of  such 
large  size  that  the  voltage  drop  in  it  will  be  much  less  than 
that  necessary  to  maintain  the  voltage  at  the  receiving  appa- 
ratus up  to  the  value  absolutely  required.     The  reason  why 
the  use  of  such  large  conductors  is  frequently  advisable  is  so 
that  the  power  loss  in  them  will  not  be  excessive.     This  situa- 
tion is  considered  more  in  detail  later. 

149.  In    Incandescent-lamp    Circuits    the    Voltage    Drop 
Allowable  varies  somewhat  with  the  character  of  the  apparatus 
supplied  and  with  other  conditions.     Where  the  incandescent 
lamps  operate  at  a  pressure  of  110  volts  or  thereabouts,  the 
circuit  conductors  should  be  so  proportioned  in  a  first-class 
installation,  that  the  pressure  at  the  lamps  will,  under  no 
condition  in  normal  operation,  vary  more  than  3  volts.     That 
is,  on  this  basis  the  maximum  permissible  voltage  drop  is  3 
volts.     Frequently,  however,  a  variation  of  4  volts  is  permitted. 
Where  the  electrical  energy  is  generated  in  the  building,  the 
entire  3  or  4  volts  drop  can  then  occur  in  the  conductors 
within  the  building.     Where  the  energy  is  generated  by  a 
central  station,  it  is  usually  customary,  with  110-volt  circuits, 
to  allow  2  volts  drop  in  the  conductors  within  the  building  and 
assume  that  there  will  be  1  or  2  volts  drop,  or  the  equivalent 


SEC.  7]     GENERAL  PRINCIPLES  OF  CIRCUIT  DESIGN          121 

thereof,  in  the  conductors  between  the  buildings  and  the 
central  generating  station. 

NOTE. — A  number  of  central-station  companies  will  not  connect  110- 
volt  installations  where  the  voltage  drop  from  the  point  of  entrance  to 
the  most  remote  lamp  in  the  inside  wiring  installation  exceeds  2  volts.  A 
few  companies  limit  the  drop  in  the  inside  wiring  installation  to  1  volt. 
Sometimes,  in  isolated  installations,  where  energy  is  produced  at  low  cost, 
a  5-volt  drop  is  allowed  on  110-volt  incandescent-lamp  circuits  but  the 
results  are  not  wholly  satisfactory.  A  5-volt  drop  is  certainly  the  upper 
limit  for  a  110-volt  incandescent-lamp  circuit.  While  the  values  above 
enumerated  apply  specifically  to  110-volt  lamp  circuits,  they  can  be 
used  proportionately  for  lamp  circuits  operating  at  other  voltages. 
Drop  is  often  expressed  as  a  percentage,  thus:  The  total  drop  on  a  circuit 
feeding  incandescent  lamps  should  never  exceed  5  per  cent,  of  the  lamp 
voltage. 

150.  A  Greater  Drop  Is  Permitted  in  Motor  Circuits  than 
in  lamp  circuits  because  motors  are  not  so  sensitive  to  varia- 
tion of  voltage.     With  motors  a  drop  exceeding  10  per  cent, 
of  the  receiver  voltage  is  seldom  advisable  and  it  is  usually 
best,  considered  from  a  standpoint  of  operation,  to  allow  a 
drop  not  in  excess  of  5  per  cent.     If  motors  and  incandescent 
lamps  are  served  by  the  same  circuit  the  drop  in  it  should  be 
limited  to  about  3  per  cent. 

151.  The  Apportionment  of  Voltage  Drop  Among  the  Dif- 
ferent Components  of  the  Circuit  will  now  be  considered.     In 
circuit  design  it  is  necessary  to  apportion  or  distribute  the 
total  drop  which  has  been  allowed  in  the  entire  circuit  between 
the  feeders,  mains,  sub-mains  and  branches.     In  incandescent 
lighting,  most  of  the  drop  is  confined  to  the  feeders  because, 
if  there  is  excessive  drop  in  the  mains  and  branches,  lamps 
located  close  together,  but  served  by  different  mains  and 
branches,  might  operate  at  decidedly  different  brilliancies. 
With  an  isolated  plant  where  energy  is  generated  on  the 
premises,  a  total  drop  of  3  volts  (for  110-volt  circuits)  may  be 
apportioned  thus:  branches,  1  volt;  mains,  one-third  of  the 
remainder;  feeders,  two-thirds  of  the  remainder.     This  will 
give  an  actual  drop  of  1  volt  in  the  branches,  0.66  volt  in  the 
mains  and  1.33  volts  in  the  feeders.*    Where  the  premises 
is  served  by  a  central  station,  the  practice  of  the  utility  con- 
cern may  allow  2  volts  drop  in  its  secondary  mains  and  the 

•  See  table  in  author's  AMERICAN  ELECTRICIANS'  HANDBOOK. 


122  CENTRAL  STATIONS  [ART.  152 

service  to  the  premises.  In  such  a  case  the  total  drop  within 
the  premises  should  not  exceed  1  to  2  volts.  Where  a  utility 
company  is  to  give  service,  it  should  be  consulted  regarding 
its  practice  in  this  respect.  Some  central-station  companies 
require  that  the  voltage  drop  in  interior  wiring  installations 
which  they  are  to  serve  should  not  exceed  a  certain  maximum. 
In  any  case  it  is  frequently  the  practice  to  allow  1  volt  drop 
for  the  branches  and  to  apportion  the  remainder  of  the  availa- 
ble drop  to  the  main  circuits  and  feeders. 

152.  The  Apportionment  of  Drop  on  2,400-volt  Distribu- 
tion Systems  is  often  made  under  the  assumption  that  the 
secondary  voltage  of  the  transformers  remains   practically 
constant.     This  will  be  found  true  in  a  well-laid-out  system, 
particularly  if  automatic  feeder  regulators  are  used. 

152A.  The  Apportionment  of  Drop  in  Motor  Circuits  may 
be  made  on  the  basis  that  1  volt  will  be  allowed  in  the  branches, 
two-thirds  of  the  remaining  drop  in  the  mains  and  one-third 
for  the  feeders.  Most  of  the  drop  should  be  confined  to  the 
mains  in  order  that  a  variation  in  load  of  one  motor  of  the 
group  will  affect  the  others  as  little  as  possible.  Where  motor 
circuits  are  fed  by  transformers  it  is  usually  assumed  that  the 
voltage  at  the  secondary  side  of  the  transformers  remains 
practically  constant.  Therefore,  all  of  the  allowable  drop  is 
apportioned  to  the  secondary  circuit.  Where  a  group  of  motors 
is  fed  by  a  main  circuit  and  branches,  the  drop  in  the  branches, 
if  they  are  not  too  long,  is  frequently  1  volt  or  less,  under 
normal  working  conditions.  The  reason  for  this  is  because 
the  National  Electrical  Code  rules  require  that  a  branch  con- 
ductor serving  a  motor  be  capable  of  safely  carrying  a  current  25 
per  cent,  greater  than  the  normal  full-load  current  of  the  motor. 

153.  The  Safe -current-carrying  Capacity  of  Wires  Should 
Always  Be  Considered  When  Designing  Circuits.— A  con- 
ductor may  have  ample  cross-sectional  area  to  convey  cur- 
rent a  given  distance  with  a  sufficiently  small  drop  in  voltage 
but  yet  may  be  so  small  that  it  will  overheat.     After  a  wire 
has  been  selected  with  reference  to  voltage  drop,  the  safe- 
current-carrying  capacity  table  (Table  154)  should  be  con- 
sulted, and  if  the  wire  first  selected  is  not  large  enough  to 
safely  carry  the  current  in  accordance  with  the  value  specified 


SEC.  7]     GENERAL  PRINCIPLES  OF  CIRCUIT  DESIGN          123 


m  Table  154,  a  wire  that  is  large  enough,  on  the  basis  of  Table 
154  values,  should  be  used. 

164.  Allowable  Carrying  Capacities  of  Wires. — These  values 
are  taken  from  the  National  Electrical  Code,  Rule  18. 


B.  &  S.  gage 
number 

Diameter  of 
solid  wire  in  mils 

Area  in  circular 
mils 

Table  A,  rubber 
insulation, 
amperes 

Table  B,  other 
insulation, 
amperes 

18 

40.3 

1,624 

3 

5 

16 

50.8 

2,583 

6 

10 

14 

64.1 

4,107 

15 

20 

12 

80.8 

6,530 

20 

25 

10 

101.9 

10,380 

25 

30 

8 

128.5 

16,510 

35 

50 

6 

162.0 

25,250 

50 

70 

5 

181.9 

33,100 

55 

80 

4 

204.3 

41,740 

70 

90 

3 

229.4 

52,630 

80 

100 

2 

257.6 

66,370 

90 

125 

1 

289.3 

83,690 

100 

150 

0 

325.0 

105,500 

125 

200 

00 

364.8 

133,100 

150 

225 

000 

409.6 

167,800 

175 

275 

200,000 

200 

300 

0000 

460.0 

211,600 

225 

325 

300,000 

275 

400 

400,000 

325 

500 

600,000 

400 

600 

600,000 

450 

680 

700,000 

500 

760 

800,000 

550 

840 

900,000 

600 

920 

1,000,000 

650 

1,000 

1,100,000 

690 

1,080 

1,200,000 

730 

1,150 

1,300,000 

770 

1,220 

1,400,000 

810 

1,290 

1,500,000 

850 

1,360 

1,600,000 

890 

1,430 

1,700,000 

930 

1,490 

1,800,000 

970 

1,550 

1,900,000 

1,010 

1,610 

2,000,000 

1,050 

1,670 

1  mil  =  0.001  in. 


124  CENTRAL  STATIONS  [ART.  155 

155.  Calculations  for  Voltage  Drop  Are  Usually  Based  on 
the  Resistance  of  a  Circular  Mil-foot  of  Commercial  Copper 
Wire  as  copper  is  the  only  metal  used  to  any  extent  for  the 
distribution  of  electrical  energy.  It  can  be  shown  that  the 
resistance  of  any  conductor  of  circular  cross-section  may  be 
computed  from  the  formula: 


(40)  R  =          -  (ohms) 

Wherein:  R  =  the  resistance  of  the  conductor,  in  ohms. 
p  =  the  resistivity,  in  ohms  per  circular  mil-foot,  of  the  metal 
composing  the  conductor.  I  =  the  length  of  the  conductor, 
in  feet,  d  =  the  diameter  of  the  conductor  in  0.001  in. 

156.  In  Practical  Wiring  Calculations  the  Resistance  of  a 
Circular  Mil-foot  of  Copper  May  Be  Taken  as  11  Ohms*  and 
since  d2  =  the  diameter  of  the  conductor,  in  0.001  in.  squared, 
that  is  d2  =  the  sectional  area  in  circular  mils,  the  above 
formula  (40)  becomes: 

(41)  R  =    U  X.|  (ohms) 

ar.  mils 


(42)  I  =       -  X  R  (feet) 

11  X  I 

(43)  dr.  mils  =  —  =  —  (circular    mils) 

K 

Wherein:  All  of  the  symbols  have  the  meanings  specified 
above. 

157.  The  Drop  or  Loss  of  Voltage  in  Any  Conductor  can  be 
most  conveniently  computed  by  using  the  Ohm's  law  formula, 
which  has  been  so  modified  that  the  expression  for  resistance 
given  in  equation  (41)  above  is  used.  Thus,  from  Ohm's  law:* 

(44)  V  =  /  X  R  (volts) 

Wherein  :  V  =  the  drop,  in  volts,  in  a  given  conductor.  /  = 
the  current,  in  amperes,  in  that  conductor.  R  =  the  resist- 
ance, in  ohms,  of  the  conductor.  Since,  however,  formula  (41) 

*  See  the  author's  AMERICAN  ELECTRICIANS'  HANDBOOK. 


SEC.  7]     GENERAL  PRINCIPLES  OF  CIRCUIT  DESIGN          125 

above  also  gives  an  expression  for  R,  it  can  be  substituted  in 
formula  (44)  above  with  this  result: 


(47) 


(48)  «_.  (feet) 

(49)  dr.  mils.  =  7  X  ^  >  (circular  mils) 

Note  that  the  symbol  Z  in  the  above  equations  stands  for 
the  double  distance  of  the  circuit.     That  is,  the  entire  length 


(*- - Double  Distance*  I  =  ?00  Ft.  - 


<  -Source  ofE.M.E  10  Lamps- 
t- N! K <  Wire;  Area =10404  Cir.  Mils 


K- IQOFt.--- ->1 

Fio.  89. — Illustrating  double  distance. 

of  the  circuit,  in  feet,  is  shown  in  Fig.  89.     See  the  following 
example. 

EXAMPLE. — The  drop  in  the  circuit  of  Fig.  89,  with  a  current  of  10 
amp.  flowing  in  the  circuit,  which  is  200  ft.  long  and  of  No.  10  wire 
(area  is  10,380  cir.  mils,  see  Table  154)  will  be,  substituting  in  formula  (46) : 

I  X  11  X  I       10  X  11  X  220 
V  =  —  —  =  —  —  =  2.1  volts, 

cir.  mils  10,380 

That  is,  the  total  drop  in  the  circuit  of  Fig.  89,  with  a  current  of  10  amp. 
flowing,  is  2.1  volts. 

158.  To  Compute  the  Voltage  Drop  in  a  Lighting  or  Power 
Circuit,  a  modified  form  of  equation  (46)  is  most  convenient. 
Since,  in  actual  installations,  the  two  side  wires  of  any  circuit 
follow  about  the  same  course  and  are  each  of  about  the  same 


126  CENTRAL  STATIONS  [ART.  158 

length,  it  is  desirable  to  use  the  single  distance  (see  Fig.  90) 
designated  by  the  symbol  L  rather  than  the  double  distance, 
designated  by  the  symbol  I.  Now,  I  (as  is  obvious  from  Figs. 
89  and  90)  equals  2  X  L.  Therefore,  substituting  2  X  L  for 
I  in  equation  (46)  we  have: 

(50)  V  =  *X  11X2  XL  (volts) 

cir.  mils 

However,  since  the  values  of  "11"  and  "2"  would  always 
appear  in  the  formula  it  is  convenient  to  multiply  them  to- 
gether once  for  all  and  then  to  use  the  value  of  "22"  in  the 

Single  Distance  =L  = 100  Ft. -. •> 

/CM/77/7. —> 
'^—Source  of£.U.F.  10  Lamps -> 

^.j -(—IP  Amp.          v  -N°/0  Wire;  Ana =10404  Cir.  Mils, 

K— 100  ft— * 

FIG.  90. — Illustrating  single  distance. 

formula  instead  of  "11  X  2."  Hence,  from  (50),  the  working 
formula  now  becomes: 

T  V  99   V    T 

(51)  V  =  .,  (volts) 

cir.  mils. 

/,.rtN  T       V  X  dr.  mils  ,  ^ 

(52)  I  =  -.  vx  , —  (amperes) 

22  X  L 

(53)  L  =  ar'™1*:  *—  (feet) 

(54)  cir.  mils   =  22x^xL  (circular  mils) 

Wherein:  V  =  the  drop  or  loss  of  potential,  in  volts,  in  the 
circuit  under  consideration.  7  =  the  current,  in  amperes, 
in  the  circuit  under  consideration.  L  =  the  single  distance, 
Fig.  90,  of  the  circuit  under  consideration.  Cir.  mils  =  the 
area  of  the  conductor  of  the  circuit,  in  circular  mils,  as  shown 
in  Table  154. 

EXAMPLE.— What  is  the  voltage  drop  in  the  circuit  of  Fig.  90?  The 
current,  /,  is  10  amp.;  the  single  distance,  L,  is  100  ft.  and  the  conductor 
is  of  No.  10  wire.  Now,  a  No.  10  wire  has,  from  Table  154,  an  area  of 


SEC.  7]     GENERAL  PRINCIPLES  OF  CIRCUIT  DESIGN          127 


10,380  cir.  mils.  SOLUTION. — Substituting  in  formula  (51):  V  = 
(1  X  22  X  L)  -5-  cir.  mils  =  (10  X  22  X  100)  -f-  10,380  =  (22,000  -^ 
10,380)  =2.1  volts.  Therefore,  the  drop  in  the  circuit  of  Fig.  90  is 
2.1  volts.  Observe  that  this  is  the  same  result  that  was  obtained  with 
the  other  form  (46)  of  the  formula  in  the  example  given  in  connection 
with  Fig.  89.  Both  examples,  obviously,  show  solutions  of  the  same 
problem  but  in  the  first  the  double  distance  I  was  used  and  in  the  sec- 
ond the  single  distance  L.  Other  examples  are  given  in  the  following 
paragraphs. 


Distribution  Center-  -> 


K- -- 200ft. -- -x 

FIQ.  91. — Finding  size  of  wire. 

159.  To  Find  the  Size  Wire  for  a  Circuit  When  the  Current, 
Length  of  Circuit  and  Allowable  Drop  are  Known  (this  relates 
specifically  to  direct  current  two-wire  circuits)  it  is,  obviously, 
merely  necessary  to  substitute  the  known  values  in  equation 
(54)  given  above. 

EXAMPLE. — What  size  wire  (Fig.  91)  could  be  used  for  a  feeder  to  carry 
a  current  of  50  amp.  from  the  main  switch  to  a  distribution  center  200' 


-Hain-WIO Wire  -  Panel  Bo*-~ * 

£    10.380  Cir.  Mils 


x - dort.  — 

Allowable  Drop  =  ?  Volts 
FIG.  92. — Circuit  from  main  to  panel  box. 


ft.  distant  measured  along  the  circuit.  The  allowable  drop  is  2  volts. 
SOLUTION.— Substitute  in  equation  (54)  thus:  Cir.  mils  =  (22  X  /  X  L) 
-=-  V  =  (22  X  50  X  200)  -=-  2  =  (220,000  +  2)  =  110,000  dr.  mils. 
Now,  referring  to  Table  154;  the  next  standard  size  wire  larger  than 
110,000  cir.  mils  is  No.  00  (which  has  an  area  of  133,100  cir.  mils). 
Hence,  No.  00  wire  should  be  used. 

160.  To  Find  the  Current  in  a  Circuit  That  Will  Cause  a 
Given  Drop  in  a  Given  Wire  of  Known  Length  (this  relates 
specifically  to  direct-current  two-wire  circuits)  formula  (52) 
may  be  utilized  as  shown  in  the  following  example. 


128 


CENTRAL  STATIONS 


[ART.  161 


EXAMPLE. — What  is  the  greatest  current  that  can  be  carried  by  the 
circuit  of  Fig.  92,  which  extends  from  a  main,  A,  to  a  panel  box,  B,  80 
ft.  distant,  with  an  allowable  drop  of  2  volts?  The  wire  is  No.  10  which 
has  (see  Table  154)  an  area  of  10,380  cir.  mils.  SOLUTION. — Substitute 
in  formula  (52)  above  thus:  7  =  (cir.  mils  X  V)  -J-  (22  X  L)  =  (10,380 
X  2)  -s-  (22  X  80)  =  (20,760  -f-  1,760)  =11.8  amp.  Therefore,  no  cur- 
rent greater  than  11.8  amp.  could  be  carried  by  the  No.  10  wire  circuit 
of  Fig.  92  without  causing  a  drop  greater  than  2  volts. 

161.  To  Find  the  Length  Circuit  That  Will  Carry  a  Known 
Current  Over  a  Conductor  of  Known  Size  With  a  Given  Drop 
(this  relates  specifically  to  direct-current  two-wire  circuits) 


Drop  Must  be  3  Volts 


•  -Single  Distance  =  t  =  ? 


-Q- 


•-N?  8  Wire  =  16,510  Cir.  Mils 


FIG.  93. — Arranging  circuit  to  have  a  given  drop. 

equation    (53)   may  be  utilized   as  shown  in   the  following 
example. 

EXAMPLE. — Some  No.  8  copper  wire  having  (from  Table  154)  an  area 
of  16,510  cir.  mils,  is  available.  It  is  desired  that  just  enough  of  it  be 
used  that  the  drop  in  it  shall  be  3  volts  when  a  current  of  30  amp.  is 
forced  through  it.  The  wires  are  to  be  arranged  between  a  switch  and  a 
group  of  lamps  as  shown  in  Fig.  93.  How  many  feet  of  circuit  must  be 
inserted  between  the  switch,  S,  and  the  lamps,  At  SOLUTION. — Sub- 
stitute in  formula  (53)  thus:  L  =  (cir.  mils  X  V)  +  (22  X  /)  =  (16,510 
X  3)  -s-  (22  X  30)  =  (49,530  -4-  660)  =  75  ft.  (almost).  The  length  of 
the  circuit,  single  distance,  would  be  75  ft.  but  the  length  of  wire  required, 
double  distance,  would  be  150  ft. 

162.  The  Formula  for  Figuring  the  Power  Loss  in  Any 
Conductor,  Either  Alternating-current  or  Direct-current, 


SEC.  7]     GENERAL  PRINCIPLES  OF  CIRCUIT  DESIGN          129 

may  easily  be  derived  from  what  has  preceded.  It  can  be 
shown:* 

(55)  P  =  I2  X  R  (watts) 

Now,  from  equation  (41):  R  =  (11  X  Z)  -f-  dr.  mils.  Then 
substituting  this  value  for  R  in  formula  (55)  the  result  is: 

(56)  P  =  I2  X    U  X.!  (watts) 

cir.  mils 

Wherein. — P  =  the  power  lost  in  the  conductor,  in  watts. 
/  =  the  current,  in  amperes  in  the  conductor.  I  =  the  length 
(double  distance)  of  the  conductor,  in  feet.  Cir.  mils  =  the 
area  of  the  conductor,  in  circular  mils. 

163.  The  Formula  for  Computing  the  Power  Loss  in  Any 
Direct-current  Two-wire  Circuit  (the  formula  also  applies  for 
a  single-phase  alternating-current  circuit)  follows  from  equa- 
tion (55)  above  and  the  fact  that  (see  Figs.  89  and  90)  double 
distance  equals  twice  single  distance.  Thus: 

(87)  P  =  PX2X11XL  =  ttX/'Xj 

cir.  mils  cir.  mils 

hence, 


(58)  I  =  (ampere8) 

(59)  L  -  <fr-aa"x'1Xf  (feet) 

(60)  cir.  mils  =  —    — ~ — -  (circular  mils) 

Wherein. — All  of  the  symbols  have  the  meanings  hereinbefore 
specified  except  that  P  =  the  power  expended  in  the  circuit 
in  watts  and  L  =  the  single  distance  of  the  circuit,  in  feet. 

164.  The  Determination  of  the  Loads  That  Will  be  Carried 
by  the  Conductors  is  the  first  operation  that  should  be  per- 
formed in  making  wiring  calculations  for  any  wiring  installa- 
tion, large  or  small.  Where  an  installation  of  any  consequence 
is  to  be  figured,  blue  prints  are  furnished  and  on  these  the  loads, 
in  amperes,  that  will  be  imposed  on  the  different  conductors 

•  See  the  author's  PBACTICAL  ELBCTBICITT. 
9 


130  CENTRAL  STATIONS  [ART.  165 

can  be  indicated  in  pencil.  If  no  plans  are  furnished  it  will 
usually  be  found  profitable,  even  if  the  installation  is  small  and 
simple,  to  make  a  pencil  sketch  of  the  wiring  layout  and  note 
on  it  the  load,  in  amperes,  that  the  conductors  must  carry. 
It  is  usually  most  convenient  to  reduce  the  loads  from  horse- 
power, watts,  etc.,  into  corresponding  ampere  values. 

NOTE. — Where  the  loads  are  thus  reduced  to  amperes,  the  equivalent 
values  are  available  for  substitution  in  the  formulas  for  computing  wire 
sizes.  Furthermore,  it  is  necessary,  in  nearly  every  case,  to  know  the 
current  each  conductor  will  carry,  so  that  one  can  be  certain  that  the 
conductor  is  sufficiently  large  to  safely  carry  it.  The  currents  taken  by 
arc  and  incandescent  lamps  and  motors  of  the  different  types  and  capaci- 
ties will  be  found  in  tables  given  in  the  author's  AMERICAN  ELECTRICIANS' 
HANDBOOK. 

165.  In  Noting  the  Ampere  Loads  on  a  Wiring  Plan,  com- 
mence with  the  branches  and  if  the  receivers  on  the  branches 
are  of  several  different  capacities,  indicate  opposite  each  the 
current  it  takes.     Then,  total  the  current  values  of  each  branch 
and  note  this  final  total  at  the  point  where  the  branch  joins 
the  larger  conductor  which  feeds  it.     Place  a  circle  around  the 
total  value  to  show  it  is  the  total.     If  all  of  the  receivers  on  a 
branch  are  of  the  same  capacity  the  aggregate  load  can  be 
readily  totaled  without  indicating  the  load  at  each  receiver. 
If  there  is  a  probability  of  the  addition  of  future  receivers, 
indicate  that  they  are  "future"  with  the  letter  "F." 

166.  National  Electrical  Code  Rules  Require  That  Motor 
Branch  Circuits  be  of  sufficiently  large  wire  that  they  will 
safely  carry  a  current  25  per  cent,  greater  than  the  normal 
current  of  the  motor.     This  has,  however,  nothing  to  do  with 
the  voltage  drop  on  mains  and  feeders,  so  the  normal  full-load 
current  of  each  motor  is  noted  on  the  plan  and  the  normal 
motor  currents  are  added  to  get  the  total  current  values.     It 
is  often  convenient  to  note  a  current  25  per  cent,  greater  than 
the  full-load  current,  in  a  square  near  each  motor  branch,  so 
that  the  wire  used  in  the  branch  can  be  readily  checked  for 
carrying  capacity. 

167.  The  Symbol  L  hi  the  Formulas  Stands  for  the  Distance 
in  Feet  to  the  Load  Center  of  the  Circuit. — This  distance  will 


SEC.  7]     GENERAL  PRINCIPLES  OF  CIRCUIT  DESIGN          131 

be  the  actual  length  if  the  load  is  concentrated  at  the  end  or 
it  will  be  the  distance  to  the  "center  of  gravity"*  if  the  load 
is  distributed.  When  found,  this  distance  is  used  as  the 
length  of  the  circuit  and  is  substituted  for  the  letter  L  in  the 
formula. 

168.  In  Measuring  Plans  a  convenient  scale  can  be  made  by 
dividing,  with  pencil  lines,  a  strip  of  drawing  paper,  possibly 
30  in.  long,  into  "feet"  divisions,  to  the  same  scale  as  that  of 
the  drawing  under  consideration.  The  number  of  feet  repre- 
sented by  each  mark  is  indicated  by  the  numeral  opposite  the 
mark.  In  use,  the  0  mark  at  one  end  of  the  scale  is  placed 
opposite  the  starting  point  of  the  circuit  and  the  paper  strip 
is  laid  along  the  circuit.  The  length  of  the  circuit,  unless  it 
is  too  long,  is  then  read  directly  from  the  strip.  Such  paper 
strips  are  very  convenient,  because  they  are  cheap,  light  and 
easily  handled  and  they  can  be  bent  to  follow  contours  of  cir- 
cuits having  irregular  courses. 

*  See  the  author's  AMERICAN  ELECTRICIANS'  HANDBOOK,  under  the  heading 
"Load  Center." 


SECTION  8 

CALCULATION  AND  DESIGN  OF  DIRECT- 
CURRENT  CIRCUITS 

169.  Direct-current    Circuit    Conductors    are    most    con- 
veniently calculated  from  formula  (54).    The  examples  given 
under  the  following  paragraph  illustrate  the  application  of  this 
equation. 

170.  The  Calculation  of  a  Direct-current  Two-wire  Current 
can  be  best  explained  by  the  consideration  of  numerical 


5  hp.  110  Volt- 
Hofor  52  Amp. 


11111 
FIG.  94. — Wire  sizes  for  an  installation. 

examples.     One  problem  showing  how  the  wire  size  for  a 
feeder  may  be  calculated  was  given  in  connection  with  Fig.  91. 

EXAMPLE. — What  size  wire  should  be  used  for  a  branch  circuit  in  which 
the  allowable  drop  is  1  volt;  the  current  is  10  amp.  and  the  distance  from 
the  start  of  the  circuit  to  the  load  center  is  60  ft.?    SOLUTION.— Substi- 
tute in  formula  (54)  thus:  Cir.  mils  =  (22  X  /  X  L)  -i-  V  =  (22  X  10 
132 


SEC.  8]  DIRECT-CURRENT  CIRCUITS  133 

X  60)  -5-  1  -  (13,200  dr.  mils.  Now  refer  to  Table  154:  a  18,200-cir. 
mil  conductor  is  larger  than  a  No.  10  wire  and  smaller  than  a  No.  8. 
As  a  rule,  the  next  larger  size  should  be  selected;  therefore,  No.  8  wire 
(which  with  rubber  insulation  will  safely  carry  35  amp.)  will  be  used. 

EXAMPLE. — In  Fig.  94  is  shown  a  diagram  of  a  distribution  installation. 
The  wire  sizes  for  each  circuit  will  be  computed  below.  The  length  of 
each  circuit,  or  the  distance  to  its  load  center,  is  indicated  above  the 
circuit  and  the  allowable  drop  in  volts  in  each  circuit  is  shown  with  the 
length.  The  total  drop  allowed  from  the  entrance  switch,  S,  to  the  last 
lamp  on  any  circuit  is  4  volts.  However,  the  total  drop  allowed  to  the 
motor,  M ,  is  5  volts.  The  current,  in  amperes,  taken  by  each  receiver 
(consuming  device)  is  indicated  opposite  the  receiver  and  the  total 
ampere  load  of  each  circuit  is  indicated  at  the  starting  point  of  the 
circuit  within  a  circle.  Each  circuit  is  designated  by  a  letter  or  group  of 
letters.  Each  component  will  be  considered  separately.  SOLUTION. — It 
will  be  assumed  that  all  of  the  conductors  will  have  rubber  insulation; 
therefore,  their  current-carrying  capacities  will  be  determined  by  the 
values  given  in  column  A  of  Table  154. 

BRANCH  ABi. 

Load  4  amp.;  distance,  20  ft.;  drop,  1  volt.  Substitute  in  formula 
(54):  Or.  mils  =  (22  X  /  X  L)  -5-  V  =  (22  X  4  X  20)  +  1  =  1,760  dr. 
mils. 

Now  referring  to  Table  154,  the  standard  size  wire  next  larger  than 
1,760  cir.  mils  is  No.  16,  which  has  an  area  of  2,583  cir.  mils.  This 
size  wire  would  be  satisfactory  were  it  not  for  the  fact  that  the  National 
Electrical  Code  prohibits  the  use  of  any  wire  smaller  than  No.  14.  Hence, 
No.  14  must  be  used  in  this  case.  (In  outdoor  service  no  copper  wire 
smaller  than  No.  8  or  No.  6  has  sufficient  mechanical  strength  to  give 
satisfactory  service.)  No.  14  rubber  insulated  wire  has  a  safe  current- 
carrying  capacity  of  15  amp.  hence,  is  amply  large  to  carry  the  4  amp. 
load  in  circuit  AB\. 

BRANCH  AB2. 

Load,  6  amp.;  distance,  40  ft.;  drop,  1  volt.  Substitute  in  formula 
(54):  Cir.  mils  =  (22  X  /  X  L)  -=-  V  =  (22  X  6  X  40)  -i-  1  =  5,280 cir. 
mils.  Referring  to  Table  154,  the  next  larger  size  wire  is  No.  12  which 
has  an  area  of  6,530  cir.  mils  and  since  it  has  a  current-carrying  capacity 
of  20  amp.  will  readily  carry  the  6  amp.  of  circuit  AB\.  Therefore,  No. 
12  is  satisfactory  and  should  be  used. 

SUB-FEEDER  AB. 

Load,  10  amp.;  distance,  30  ft.;  drop,  1  volt.  Substitute  in  formula 
(54):  Cir.  mils  =  (22  X  /  X  L)  -s-  V  -  (22  X  10  X  30)  +  1  =  6,600 
dr.  mils.  Use  No.  10  wire,  which  has  an  area  of  10,380  cir.  mils  and 
which  will  safely  carry  24  amp. 

BRANCH  AA\. 

Load,  52  amp.   (see  Table  in  AMERICAN  ELECTRICIANS'  HANDBOOK 


134  CENTRAL  STATIONS  [AKT.  170 

for  currents  taken  by  motors  of  different  horse-power  voltage) ;  distance, 
30  ft.;  allowable  drop,  2  volts.  Substitute  in  the  formula  (54):  Cir. 
mils  =  (22  X  7  X  L)  +  V  =  (22  X  52  X  30)  H-  2  =  (34,320  -r  2)  = 
17,160  dr.  mils.  Referring  to  Table  154,  No.  6  wire,  which  has  an  area  of 
26,250  cir.  mils,  is  the  next  largest  size.  This  wire  would  carry  the 
current  of  the  motor  with  much  less  than  2  volts  drop.  However,  since 
it  is  specified  in  the  National  Electrical  Code  that  branch  circuits  to  motors 
must  be  capable  of  safely  carrying  a  current  at  least  25  per  cent,  greater 
than  the  normal  full-load  current  of  the  motor,  a  wire  must  be  selected 
that  will  safely  carry:  52  X  1.52  =  65  amp.  Therefore,  No.  4  wire 
mu&«  be  used  for  this  branch,  which  will  safely  carry  70  amp. 

BRANCH  AAt. 

Load,  5  amp.;  distance,  20  ft.;  drop,  1  volt.  Substitute  in  the  formula 
(54):  Cir.  mils  =  (22  X  /  X  L)  -i-  V  =  (22  X  5  X  20)  -=-  1  =  2,200  cir. 
mils.  In  Table  154  it  is  shown  that  the  area  of  a  No.  16  wire  is  2,583 
cir.  mils  so  this  size  would  be  satisfactory  insofar  as  drop  in  voltage  is 
concerned.  However,  as  above  outlined,  wires  smaller  than  No.  14 
are  not  permitted  in  ordinary  wiring.  Hence,  No.  14  must  be  used. 
No.  14  will  safely  carry  15  amp.,  hence  is  amply  safe  for  the  5  amp.  load 
of  branch  AA2. 

BRANCH  A  A  3. 

Load,  5  amp.;  distance,  50  ft.;  drop,  1  volt.  Substitute  in  the  formula 
(54) :  Cir.  mils  =  (22  X  /  X  L)  -J-  V  =  (22  X  5  X  50)  -J-  1  =  5,500  cir. 
mils.  Again  referring  to  Table  154,  a  No.  12  wire,  which  is  plenty  large 
enough  to  carry  the  current,  should  be  used. 

BRANCH  AA4. 

Load,  3  amp.;  distance,  40  ft.;  drop,  1  volt.  Substitute  in  formula 
(54):  (Cir.  mils)  =  (22  X  /  XL)  •*-  F  -  (22  X  3  X  40)  •*•  1  -  2,640 
cir.  mils.  By  consulting  Table  154  it  is  found  that  a  No.  14  wire  should 
be  used. 

SUB-FEEDER  AA. 

Load,  65  amp.;  distance,  20  ft.;  drop,  1  volt.  Substitute  in  formula 
(54):  Cir.  mils  =  (22  X  /  X  L)  -H  V  =  (22  X  65  X  20)  -^  1  =  28,600 
cir.  mils.  Now,  referring  to  Table  154,  No.  5  wire,  which  has  an  area  of 
33,100  cir.  mils,  would  satisfy  the  conditions  as  to  drop.  However, 
on  sub-feeder  A  A,  is,  as  indicated  in  Fig.  94,  65  amp.  Therefore,  it 
would  be  necessary  to  use  a  No.  4  conductor  which  has  a  safe  current- 
carrying  capacity  (Table  154)  of  70  amp.  Furthermore,  it  should  be 
noted  that  it  was  necessary  to  use  No.  4  wire  for  branch  AAi.  Therefore, 
a  wire  at  least  as  large  as  No.  4  wire  will  probably  be  required  for  sub- 
feeder  A  A.  In  some  localities  the  inspector  might  require  the  installa- 
tion of  No.  3  for  sub-feeder  A  A. 

FEEDER  A. 

Load,  75  amp.;  distance,  80  ft.;  drop,  2  volts.  Substitute  in  formula 
(54):  Cir.  mils  =  (22  X  /  X  L)  -{-  V  =  (22  X  75  X  80)  •*-  2  =  66,000 


SEC.  8] 


DIRECT-CURRENT  CIRCUITS 


135 


dr.  mils.  By  referring  to  Table  154  it  is  evident  that  a  No.  2  wire,  which 
has  an  area  of  66,370  cir.  mils,  may,  since  it  will  safely  carry  90  amp. 
(the  load  is  but  75  amp.)  be  used. 

EXAMPLE. — What  size  wire  should  be  used  for  the  line  shown  in  Fig. 
95,  which  supplies  a  40-h.p.,  220-volt  motor?  The  normal  current  of  this 
motor,  as  obtained  from  a  table  of  motor  currents,  is  150  amp.  All  of 
the  wiring  is  assumed  to  be  supported  on  a  pole  line  or  exposed.  Hence, 
weather-proof  insulated  copper  wire  will  be  used  on  the  pole  line  and  slow- 
burning  insulated  wire  within  the  buildings.  SOLUTION. — Load,  150 
amp.;  distance,  500  ft.;  allowable  drop  =  0.044  X  220  =  10  volts,  ap- 
proximately. Substitute  in  formula  54:  Cir.  mils  =  (22  X  /  X  L)  -J-  V 
=  (22  X  150  X  500)  -=-  10  =  1,650,000  -=-  10  =  165,000  cir.  mils.  Re- 
ferring to  Table  154,  the  next  largest  standard  size  wire  is  No.  000,  which 
has  an  area  of  167,000  cir.  mils  and  will  safely  carry  272  amp.  with  either 
slow-burning  or  weather-proof  insulation.  Hence,  No.  000  is  the  wire 
size  that  should  be  used. 


Jakes  ISO 

—  —  Amperes- 

Allowable  Drw  4.4%  =  .044  x  220  =  9.68  or  Say  10  Volts. 

FIG.  95. — Size  wire  for  motor  line. 


171.  The  Calculations  for  Direct-current  Three-wire  Cir- 
cuits are  made  in  essentially  the  same  manner  as  are  those  for 
direct-current  two- wire  circuits.     With  the  "balanced"  three- 
wire  circuits  no  current  flows  in  the  neutral  wire.     In  practice 
circuits  should  be  very  nearly  balanced  and  in  making  wiring 
calculations  it  is  usually  assumed  that  they  are  balanced 
unless  there  is  obviously  a  great  unbalance. 

172.  The   Process  in  Determining   Conductor   Sizes   for 
Three -wire  Circuits  is  about  as  follows:  The  first  step  is  to 
ascertain  the  current  which  will  flow  in  the  outside  wires. 
This  value  is  obtained  in  practice  by  adding  together  the  cur- 
rents taken  by  all  of  the  receivers  which  are  connected  be- 
tween the  neutral  and  the  outside  wires  and  divide  the  sum 
by  2,  as  indicated  in  Fig.  96.     Then,  to  this  value  are  added 
the  currents  taken  by  the  receivers,  if  there  are  any,  which  are 
connected  across  the  outside  wires.     The  sum  of  these  values 


136 


CENTRAL  STATIONS 


[ART.  172 


is  then  taken  as  the  total  current.  The  computation  is  then 
made  by  the  same  method  as  for  any  two-wire  circuit.  The 
neutral  wire  is  ordinarily  disregarded  in  the  calculation  be- 
cause it  is  usually  assumed  that  it  carries  no  current.  The 


(Siffe  Circuits  =  (40  Amp,  f  50  Amp.  +10  Amp.)  +  2 
[outside  Circuit 


(  100  Amp.}  -r  2  =50Amp. 
=30  Amp. 


Total  Assumed/  Load  =-80  Amp. 

FIG.  96.  —  Illustrating  how  the  ampere  load  on  an  unbalanced  three-wire 
circuit  may  be  estimated. 

neutral  may*  be  made  smaller  than  the  outside  wires  but  is 
frequently  made  of  the  same  cross-sectional  area.  The  drop, 
V,  in  the  formula  is  the  drop  in  the  outside  wires  and  is  two 


—SOfJ.  -4  Volts— 


FIG.  97. — Size  wire  for  three-wire  main. 

times  the  drop  of  each  receiver  between  the  neutral  and  an 
outside  wire. 

EXAMPLE. — What  size  wire  should  be  used  for  the  three-wire  sub-feeder 
of  Fig.  97.  Allowable  drop  is  4  volts,  length  of  sub-feeder,  50  ft.  and  total 
110-volt  load  20  amp.  SOLUTION. — Although  the  load  is  not  balanced  on 

*  See  the  author's  AMERICAN  ELECTRICIANS'  HANDBOOK. 


SBC.  8]  DIRECT-CURRENT  CIRCUITS  137 

the  two  sides  of  the  circuit,  it  would  be  assumed  in  practical  wiring  cal- 
culations that  it  is  balanced.  Actually,  with  the  loads  as  shown  in 
Fig.  97,  11  amp.  would  flow  in  the  upper  outside  wire,  AB,  9  amp.  in 
the  lower  outside  wire,  EF  and  2  amp.  in  the  neutral,  CD.  In  practice, 
it  would  be  assumed  that  one-half  of  the  total  load  (between  neutral  and 
outside  wires),  or  (5  +  6  +  4  +  5)  -*-  2  =  10  amp.,  would  flow  in 
each  outside  wire.  Hence:  load,  10  amp.;  distance,  50  ft.;  drop,  4  volts. 
Substitute  in  formula  (54):  Cir.  mils  =  (22  X  7  X  L)  -e-  V  =  (22  X  10 
X  50)  -:-  4  =  2,750  dr.  mils.  From  Table  154  it  is  evident  that  the 
next  larger  size  than  one  having  2,750  cir.  mils  area  is  a  No.  14  which  has 
an  area  of  4,107  cir.  mils  and  which  will  safely  carry  12  amp.  Hence, 
No.  14  is  the  wire  to  use. 

173.  The  Actual  Voltage  Drop  in  an  Unbalanced  Three- 
wire  Circuit  may  be  calculated  by  using  the  formula  (46). 
The  operation  will  be  illustrated  with  an  example. 

EXAMPLE. — Consider  a  circuit  loaded  as  shown  in  Fig.  98.  The  circuit 
is  373  ft.  long,  each  of  the  conductors  is  of  No.  14  wire  (area,  4,107  cir. 
mils)  and  the  load  is  unbalanced,  the  receivers  on  one  side  of  the  circuit 
taking  10  amp.  while  those  on  the  other  side  take  only  1  amp.  What  is 
the  voltage  drop  in  the  conductors  and  what  is  the  voltage  at  the  re- 
ceivers? SOLUTION. — The  current  in  each  wire  is,  obviously,  that  in- 
dicated in  Fig.  98.  To  find  the  drop  in  volts  in  each  wire,  substitute  in 
formula  (46)  thus: 


K     —  

I 

37JU  

A, 

*    *    °A 

•    100  V. 

IOAmp.--->            ? 

f     t 

•k* 

urnm 

\   100V. 

-t---9Amp.         '.-us  14  wirt  ,-"* 
/'  4100  Cir.  Mils 
i 

1  1* 

j.        V 

IZsil 

FIG.  98. — Drop  in  unbalanced  three-wire  circuit. 

FOR  WIRE  AAi. 

V  =  (11  X  /  X  i)  -s-  cir.   mils  =  (11  X  10  X  373)  -=-  4,107  =  10  volts. 

FOR  WIRE  BBi. 

V  =  (11  X  /  X  I)  -s-  cir.  mils  =  (11  X  9  X  373)  -=-4,107  =  9  volts. 

FOR  WIRE  CCi. 

V  =  (11  X  /  X  1)  +  cir.  mils  =  (11  X  1  X  373)  -=-  4,107  =  1  volt. 

The  voltage  across  the  two  outside  wires  A  iCi  at  the  end  of  the  circuit 
is  obtained  by  subtracting  the  sum  of  the  drops  in  the  two  outside  wires 
from  the  impressed  voltage,  thus:  200  -  (10  +  1)  =  200  -  11  =  189 
volts.  Therefore,  189  volts  is  the  pressure  across  the  two  outside  wires 
at  the  end  of  the  circuit.  The  voltage  at  the  end  of  the  circuit  between 


138 


CENTRAL  STATIONS 


[ART.  174 


the  neutral  and  the  upper  outside  wire  is  (see  Fig.  98)  81  volts  and  the 
pressure  between  the  neutral  and  the  lower  outside  wire  is  108  volts. 
The  method  of  obtaining  these  voltages  will  be  made  clear  by  a  considera- 
tion of  Fig.  99  which  illustrates  the  same  general  problem  as  Fig.  98  and 
which  is  further  discussed  in  a  following  article. 

174.  To  Insure  That  Three-wire  Circuits  Will  Be  Balanced 
as  Nearly  as  Is  Feasible,  the  lamps  and  other  devices  served 
by  the  circuit  should  be  divided  between  the  two  sides  of  the 
system  so  that  the  load  on  the  two  side  circuits  will  be  equal, 
for  full  capacity  or  for  any  fraction  thereof.  For  this  reason, 


Circuit  Diagram  Voltage  Drop  Diagram 

I -Circuit  Balanced 

FIQ.  99. — Effect  of  unbalance  on  a  three-wire  circuit. 

three  wires  should  be  carried  to  any  location  where  a  con- 
siderable amount  of  energy  is  required.  Three  wires  should 
be  carried  to  every  building  in  the  case  of  an  outdoor  dis- 
tribution and  to  every  distribution  center  in  the  case  of 
distribution  between  buildings.  In  case  many  lamps  are  to 
be  lighted  at  the  same  time  they  should,  preferably,  be 
controlled  by  three-way  switches. 

NOTE. — Although  every  precaution  may  be  taken  to  insure  equal 
loading  on  the  two  side  circuits,  it  is  possible  that  the  balance  cannot 
be  maintained  in  practice.  For  example,  it  may  occur  that  a  great  many 


SEC.  8]  DIRECT-CURRENT  CIRCUITS  139 

lamps  are  lighted  on  one  side  circuit  of  a  three-wire  system  and  very  few 
on  the  other  side.  In  the  event  of  such  a  contingency  the  drop  in  voltage 
would  be  about  twice  its  normal  value  on  the  side  circuit  having  the 
larger  number  of  lamps  connected  to  it  and  the  voltage  across  the  lamps 
feeding  from  that  side  circuit  would  be  correspondingly  decreased. 
Simultaneously,  the  voltage  across  the  lamps  on  the  other  side  circuit 
might  be  raised.  See  Fig.  99,  which  shows  an  exaggerated  example. 
The  probability  of  a  condition  such  as  that  outlined  in  Fig.  99,77  is,  par- 
ticularly in  large  systems,  remote. 

175.  It  is  Not  Sufficient  in  a  Three-wire  System  to  Have 
Equal  Numbers  of  Lamps  on  the  Two  Sides. — They  should 
also  be  distributed  in  approximately  the  same  manner.  What 


* 4~  100  Amp.-- - 

FIQ.  100. — Effect  of  nonuniformly-distributed  load. 

may  occur  if  the  distribution  of  the  lamps  is  not  systematical 
is  indicated  in  the  following  example. 

EXAMPLE.— With  a  group  of  lamps  requiring  100  amp.  (Fig.  100) 
connected  between  the  +  and  the  —  wire  at  one  point,  AB,  and  an  equal 
connection  between  the  neutral  and  the  —  wire,  at  the  location  CD 
some  distance  away,  there  would  be  a  current  of  100  amp.  flowing 
in  the  neutral  wire  between  point  B  and  C.  This  heavy  current  in  the 
neutral  would  involve  considerable  extra  drop  although  the  system 
would  at  the  generating  station  appear  to  be  operating  in  perfect  balance. 
In  practice,  this  local  unbalancing  of  the  three-wire  system  is  one  of  the 
chief  causes  of  variation  of  voltage.  Hence,  the  possibility  of  its  occurring 
should  be  minimized  by  intelligently  distributing  this  load  on  both  the 
side  circuits  of  the  system.  It  is  because  of  this  possibility  that  it  is 
desirable  to  carry  the  three  wires  to  every  location  where  considerable 
amount  of  energy  is  required. 


SECTION  9 

CALCULATION  AND  DESIGN  OF  ALTERNATING- 
CURRENT  CIRCUITS 

176.  There  Are  Certain  Factors  Which  Affect  the  Com- 
putation of  Alternating-current  Circuits  which  are  not  en- 
countered with  direct-current  circuits.     The  phenomena  to 
which  these  effects  are  due  are  (among  others):  (1)  power 
factor,  (2)  inductance,  (3)  permittance  or  capacitance,  and 
(4)  skin  effect.*     Where  the  circuits  are  not  of  great  length 
it  is  not  necessary  to  consider  these  effects.     But,  where  the 
circuits  are  long  they  may  be  of  considerable  consequence. 
Permittance  need  seldom  be  considered  except  with  rather- 
high-voltage  circuits.     Hence,  these  permittance  effects  are 
not  treated.   The  method  for  designing — that  is,  for  computing 
the  wire  sizes  for  alternating-current  circuits  of  different  char- 
acteristics— will  be  outlined  in  following  articles. 

177.  Power  Factors  of  the  Apparatus  or  Equipment  which 
the  circuit  serves  must  often  be  known  before  the  circuit  can 
be  effectively  designed.     If  the  exact  power  factor  is  not  known 
or  cannot  be  obtained  from  the  manufacturer  of  the  apparatus 
in  question,  approximate  values f  can  be  used.     The  power 
factor  of  the  load,  if  it  be  other  than  100  per  cent.,  may  affect 
the  voltage  drop  in  the  line  considerably.     This  fact  is  brought 
out  in  a  number  of  the  examples  which  follow. 

NOTE. — For  ordinary  wiring  calculations  where  more  definite  data 
are  lacking  it  can  be  assumed  that  the  power  factor  of  loads  will  be  about 
as  follows:  Incandescent  lighting  only,  from  100  per  cent,  down  to  95 
per  cent.;  incandescent  lighting  and  motors,  85  per  cent.;  motors  only, 
80  per  cent. 

178.  The  Effect  of  Line  Reactance  must  also  be  given  con- 
sideration.    Practically  all  alternating-current  circuits  have 

•  These  phenomena  are  discussed  and  explained  in  more  detail  in  the  author's 
PRACTICAL  ELECTRICITY. 

t  See  the  author's  AMERICAN  ELECTRICIANS'  HANDBOOK  for  a  comprehensive  list  of 
operating  power  factor  of  different  kinds  of  apparatus. 
140 


SEC.  9]  ALTERNATING-CURRENT  CIRCUITS  141 

some  reactance  due  to  electromagnetic  inductance*  (see  Tables 
190A  and  1905  for  reactance-drop  values).  The  effect  of 
line  reactance  is  to  cause  a  drop  in  voltage  somewhat  similar 
to  that  caused  by  resistance.  Where  all  of  the  wires  of  a  cir- 
cuit, two  wires  for  a  single-phase  and  three  wires  for  a  three- 
phase  circuit,  are  carried  in  the  same  conduit  or  where  the  wires 
are  separated  less  than  an  inch  between  centers,  the  effect  of 
inductive  reactance  can  ordinarily  be  neglected.  Where  the 
circuit  conductors  are  larger  than  say  No.  2  wire  and  separated 
from  one  another  by  more  than  a  few  inches,  the  effect  of  in- 
ductive reactance  in  the  line  circuit  may  increase  the  voltage 
drop  considerably  over  that  drop  which  is  due  to  resistance 
alone.  In  designing  circuits,  every  circuit  which  is  long  or  the 
conductors  of  which  are  of  large  cross-sections  or  widely  sepa- 
rated should  be  investigated  for  line  reactance  drop  by  utiliz- 
ing the  methods  outlined  in  Art.  190. 

179.  The  Line  Reactance  of  Aerial  Circuits  on  Pole  Lines 
where  the  wires  are  widely  separated,  is  apt  to  be  relatively 
large.     Line  reactance  increases  as  the  size  of  the  wire  increases 
and  as  the  distance  between  wires  increases.     These  state- 
ments may  be  verified  by  referring  to  the  values  in  Tables 
190A  and  1905. 

180.  Line  or  Circuit  Reactance  May  Be  Reduced  in  Two 
Ways. — One  of  these  is  to  diminish  the  distance  between  wires. 
The  extent  to  which  the  reactance  may  be  diminished  by  this 
method  is  limited,  in  the  case  of  the  pole  line,  to  the  least 
separation  permissible  between  conductors,  due  consideration 
being  given  to  the  separation  required  for  insulation  and  to 
the  separation  necessary  to  prevent  the  wires  from  swinging 
together  in  the  middle  of  a  span.     In  inside  wiring  knob-and- 
cleat  work,  the  minimum  separation  between  conductors  is 
limited  to  the  minimum  spacing  specified  by  the  National 
Electrical  CWe.f    Where  the  conductors  are  in  conduit  they 
then  lie  so  close  together  that  the  factor  of  inductive  reactance 
with  them  is  ordinarily  negligible. 

The  other  way  of  reducing  line  reactance  is  to  divide  the 

•  See  the  author's  PRACTICAL  ELECTRICITY  for  a  discussion  of  electromagnetic  in- 
ductance and  reactance. 

t  See  the  author's  WIRING  FOR  LIGHT  AND  POWER. 


142 


CENTRAL  STATIONS 


[ART.  181 


copper  into  a  greater  number  of  circuits  and  to  arrange  these 
circuits  so  that  there  is  no  inductive  interaction.  Thus,  in 
Fig.  101  the  circuit  of  II  will  have  less  inductive  reactance 
than  the  circuit  of  7,  although  it  has  the  same  total  circular 
mils  area,  because  it  is  subdivided.  How  and  why  subdivision 


..-•From  Source  of  Energy  Supply- 
t.;IOO.OOO  Cir.  Mils  Conduct, 


I -Not  Subdivided 


500.000  Cir.  Mils  Condud 


Circuit  B-~* 
H-  Subdivided 


FIG.  101. — Showing  how  a  conductor  may  be  subdivided. 

decreases  the  reactance  of  a  circuit  will  be  evident  from  a  con- 
sideration of  some  of  the  numerical  examples  which  follow. 

NOTE. — Voltage  drop  in  lines,  due  to  inductive  reactance,  is  best 
diminished*  by  subdividing  the  copper  or  by  bringing  the  conductors 
close  together.  It  is  little  affected  by  changing  the  size  of  the  conductor. 

181.  The  Arrangement  of  Conductors  in  Polyphase  Cir- 

cuits f  will  next  be  given  con- 
sideration. The  directions  for 
the  calculation  of  polyphase 
circuit  conductors  which  are 
given  in  following  articles,  hold 
only  with  certain  arrangements 
of  the  circuit  conductors. 
These  arrangements  are,  how- 
ever, the  ones  which  ordinarily 
are,  or  which  may  be  readily 
adopted.  These  conditions 
need  not  be  considered  where 
the  circuit  in  question  is  short 

and,  in  general,  they  need  not  be  considered  in  interior  wiring 
circuits.  They  become  of  importance,  however,  with  circuits 
extending  possibly  for  a  distance  of  over  a  mile. 

182.  The  Two  Circuits  of  a  Two-phase  Transmission  Should 
Be  so  Arranged  That  There  Is  No  Inductive  Interaction,  f— 

*  Ralph  Mershon. 

t  Westinghouse  Electric  &  Manufacturing  Company  publication. 


FIG.      102. — Arrangement 


two- 


phase  conductors  on  cross  arms. 


SEC.  9] 


ALTERNATING-CURRENT  CIRCUITS 


Such  an  arrangement  may  be  effected  by  either  of  the  two 
methods  shown  respectively  in  Figs.  102  and  103.  Fig.  102 
shows  the  two  wires,  a  and  a,  of  one  circuit  and  the  other 
two,  b  and  6,  of  the  other  circuit  at  the  opposite  ends  of  the 


Transposition-  -  - ,_ 


FIG.  103. — Arrangement  of  two-phase   conductors  on   crossarm. 

diagonals  of  a  square.  With  such  an  arrangement  there  is  no 
inductive  interaction  between  the  two  circuits  since  none  of 
the  flux  lines  due  to  one  of  the  circuits  can  cut  the  other. 
Fig.  103  shows  the  two  circuits  side  by  side  (they  may  be  in 

any  other  relative  position,  pro- 
vided it  is  preserved  through- 
out) and  the  wires  of  the  circuit, 
b  and  6,  interchanged  or  trans- 
posed at  their  middle  point. 
Such  an  arrangement  fulfills  the 
requirements  since  all  of  the 
linkages  from  a  and  a  to  b  and 
6  and  from  b  and  b  to  a  and  a 
in  one-half  of  the  transmission 
are  exactly  offset  by  the  same 
number  of  opposite  linkages  in 
the  other  half  of  the  transmis- 
sion. 

183.  The  Arrangement  of  the 
Three  Wires  of  a  Three-phase 

Fio.  104. — Symmetrical  arrangement  _,  .  ,        ,,     ,  , 

of  conductors,  three-phase  circuit.       IransmiSSlOn    SnOUla     DC    SUCh 

that  they  are  symmetrically  re- 
lated. Figs.  104  and  105  show  two  methods  of  effecting  this 
arrangement.  In  Fig.  104  each  of  the  three  wires  is  at  a 
corner  of  an  equilateral  triangle.  In  Fig.  105  all  three  of  the 
wires  are  on  the  same  crossarm  and  they  are  twice  transposed. 


144  CENTRAL  STATIONS  [AKT.  184 

One  transposition,  A,  is  at  one-third  of  the  transmission  dis- 
tance and  the  other,  B,  at  two-thirds  of  the  transmission  dis- 
tance. In  Fig.  104,  since  each  of  the  wires  carries  the  same 
current  and  because  of  the  symmetrical  arrangement  of  the 
conductors,  the  inductive  interaction  between  any  one  wire 
and  the  remaining  two  is  the  same  regardless  of  which  wire  is 
considered.  With  the  arrangement  of  Fig.  105  the  same  con- 
dition holds  and  can  be  verified  by  tracing  the  positions  of  the 
wires  throughout  their  lengths. 


B 
FIG.  105. — Arrangement  of  conductors  on  crossarm. 

NOTE. — The  arrangement  of  Fig.  105  may  be  used  where  all  of  the 
power  is  transmitted  to  the  end  of  the  line.  Where  all  of  the  power  is 
transmitted  to  the  end  of  the  line,  only  two  transpositions,  A  and  B,  as 
shown,  are  necessary.  But  if  power  is  tapped  off  at  intermediate  points 
and  perfect  neutralization  of  inductive  interaction  is  desired  the  wires 
should  be  interchanged  as  shown  in  Fig.  105  between  locations  at  which 
"tap  offs"  to  the  line  are  made.  That  is,  there  should  be  two  transposi- 
tions between  the  generating  station  and  the  first  tap  off;  two  between 
the  first  and  second  tap  off,  etc. 

184.  Where  the  Triangular  Arrangement  of  Three-phase 
Conductors  Is  Employed  the  Wires  May  Be  Interchanged  or 
Transposed. — This  is  unnecessary,  however,  unless  it  is  im- 
possible to  arrange  the  wires  in  a  triangle  which  is  practically 
equilateral  or  unless  there  are  two  or  more  circuits  running 
parallel  to  one  another  and  it  is  desired  to  have  them  induc- 
tively independent.  Where  it  is  desired  that  the  parallel  cir- 
cuits be  inductively  independent  they  can  be  disposed  as  sug- 
gasted  in  Fig.  106.  This  illustration  shows  a  top  view  of  the 
triangular  arrangement.  The  first  circuit,  7,  runs  straight 


SEC.  9]  ALTERNATING-CURRENT  CIRCUITS  145 

through.  The  second,  77,  is  interchanged  twice.  The  third, 
777,  is  interchanged  eight  times  and  the  fourth  would  be  inter- 
changed 26  times,  etc.  If  it  is  necessary  to  interchange  the 
first  circuit  because  of  any  inequality  in  the  sides  of  the  tri- 
angle, II  must  be  taken  as  the  first  circuit,  III  as  the  second, 
etc.  Also  77  and  777  of  Fig.  106  may  be  followed  in  trans- 
posing three-phase  circuits,  the  three  wires  of  which  are  on 
the  same  crossarm,  as  in  Fig.  105.  Then  77  is  the  first  circuit 
and  777  is  the  second  circuit  and  so  on. 


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m 

FIG.  106. — Transposition  of  three-phase  circuits. 

185.  If  an  Electrically  Symmetrical  Arrangement  Is  Not 
Employed  (that  is,  unless  one  of  the  arrangements  of  conduc- 
tors described  in  the  articles  just  preceding  is  not  utilized)  the 
unbalanced  inductive  reactance  which  will  result  will  cause 
an  unbalancing  of  the  system.  Such  an  unbalancing  will  be 
of  little  consequence  in  a  short  system  but  in  a  long  trans- 
mission it  may  cause  considerable  annoyance.  For  example, 
if  the  four  wires  of  a  two-phase  transmission  or  the  three 
wires  of  a  three-phase  transmission  are  strung  straight  through 
on  the  same  crossarm  without  transposition  unbalancing  of 
the  system  will  result. 

NOTE. — The  rule  given  in  Art.  198  for  calculating  a  three-phase  line 
applies  closely  but  not  exactly  to  Fig.  105,  for  the  reason  that  the  ar- 
rangement of  Fig.  105  cannot  be  exactly  replaced  by  two  single-phase 
circuits  the  wires  of  which  are  the  same  distance  apart  as  the  adjacent 
wires  of  Fig.  105.  No  two  wires  in  Fig.  105  are  the  same  distance  apart 

10 


146  CENTRAL  STATIONS  [ART.  186 

throughout  their  length.  They  are  at  one  distance  apart  for  two-thirds 
of  the  length  and  twice  that  distance  for  the  remaining  one-third.  The 
equivalent  single-phase  circuit  must,  therefore,  have  between  wires  a 
distance  intermediate  between  that  of  adjacent  and  extreme  wires  in 
Fig.  105. 

EXAMPLE. — Consider  a  three-phase  line  of  which  the  adjacent  wires 
are  18  in.  apart.  The  equivalent  single-phase  circuits  must  have  their 
wires  apart  a  distance  intermediate  between  18  in.  and  36  in.  What 
this  distance  is  can  be  determined  by  referring  to  a  table  of  reactances. 
Consider  a  No.  0  wire  and  a  frequency  of  60  cycles.  In  Table  1904  the 
constant  for  an  18-in.  separation  is  0.228.  That  for  36  in.  is  0.259. 
Therefore,  the  constant  of  the  equivalent  single-phase  circuit  is :  (0.228  -f- 
0.228  +  0.259)  -r-  3  =  0.238,  which  corresponds  to  a  spacing  distance  of 
about  22  in.  This  shows  one  advantage  which  the  triangular  arrange- 
ment has  over  that  of  Fig.  105,  because  for  the  same  distance  between 
adjacent  conductors,  the  reactances  with  the  triangular  arrangement 
(Fig.  104)  is  less  than  with  that  of  Fig.  105.  If  an  accurate  solution  is 
necessary,  with  an  arrangement  like  that  of  Fig.  105,  the  average  con- 
stant for  any  two  wires  must  be  taken  in  calculating  the  reactance  volts. 

186.  In  Calculating  Wire  Sizes  for  Single-phase  Alternating- 
current  Incandescent-lighting  Interior  Circuits  formula  (54) 
is  ordinarily  used.  That  is,  dr.  mils  =  (22  X  /  X  L)  +V 
may  be  applied  in  precisely  the  same  way  as  if  the  circuit  were 
a  direct-current  circuit.  The  result  which  this  formula  will 
give  is  strictly  accurate  (assuming  11  ohms  is  the  resistance  of 
a  circular  mil-foot  of  copper)  where  the  load  is  non-inductive. 
That  is,  where  its  power  factor  is  100  per  cent,  and  where  the 
line  or  circuit  to  the  load  has  no  reactance.  The  results  ob- 
tained by  using  formula  are  not  theoretically  accurate  for 
actual  alternating-current  circuits,  because  the  effects  of  the 
power  factor  of  the  lamp  load  and  of  the  line  reactance  are 
not  considered. 

NOTE. — Experience  has  shown  that  the  results  obtained  by  using  for- 
mula (54)  are  sufficiently  accurate  for  practical  circuit  design  for  wiring 
small  and  medium-sized  residences,  stores,  factories  and  the  like,  for  in- 
candescent lighting.  Where  the  conductors  are  carried  in  conduit,  the 
results  will  be  quite  accurate  for  any  size  wire  used  in  practice.  Formula 
(54)  can  always  be  used  with  safety  for  computing  branch  circuits 
for  interior  incandescent  lighting  or  where  mam  or  feeder  circuits,  com- 
posed of  conductors  of,  say,  larger  than  No.  2  wire  and  separated  from 
one  another  more  than  a  few  inches,  are  being  designed.  The  wire  size 
indicated  by  the  above  formula  should  be  checked  by  the  theoretically 


SEC.  9]  ALTERNATING-CURRENT  CIRCUITS  147 

accurate  method  outlined  in  a  following  paragraph.  From  this  it  can 
be  ascertained  whether  the  voltage  drop  in  the  particular  case  under 
consideration  will  be  excessive. 

EXAMPLE. — Any  of  the  examples  given  in  the  preceding  articles  for 
direct-current,  two-wire  or  three-wire  circuits  (for  incandescent  lamps 
only,  not  for  motors)  may  be  taken  as  examples  of  single-phase  alter- 
nating-current circuits  if  it  be  assumed  that  the  lines  have  no  reactance. 
As  noted,  this  is  a  safe  assumption  for  ordinary  low-voltage  open-wire 
or  conduit  interior  incandescent  lighting  circuits. 

187.  The  Method  of  Computing  the  Wire  Size  for  a  Single- 
phase  Alternating-current  Circuit  Where  the  Line  Has  No 
Reactance  or  may  be  assumed  to  have  no  reactance,  will  now 
be  considered.  For  an  approximate  solution  formula  (54)  may 
be  used.  This  will  always  give  a  result  which  is  on  the  safe 
side  provided  the  line  has  practically  no  reactance.  How- 


Fio.  107.  —  Wire  size  for  A.C.  circuit  (no  line  reactance). 


ever,  in  applying  formula  (54)  for  alternating-current  cir- 
cuits, the  value  of  7,  in  amperes,  must  be  the  actual  current 
which  flows  in  the  alternating-current  line.  That  is,  it  is  the 
current  stamped  on  the  nameplate  of  the  machine  or  device 
served,  or  this  value  for  /  may  be  obtained  by  dividing  the 
actual  watts  taken  by  the  voltage  times  the  power  factor  or: 
7  =  (watts)  -i-  (voltage  X  power  factor). 

EXAMPLE.  —  What  size  wire  should  be  used  for  the  open-wire  motor 
circuit  of  Fig.  107?  The  circuit  is  400  ft.  long  and  serves  a  30-h.p., 
alternating-current  motor  which,  as  rated  on  the  nameplate,  is  taking 
102  amp.  Since  the  circuit  wires  are  very  close  together  there  is  practi- 
cally no  line  reactance.  The  allowable  drop  is  12  volts.  SOLUTION.  — 
Current  =  102;  distance  =  400  ft.;  drop  =  12  volts.  Substitute  in 
formula  (54): 

..         22X/X  L      22  X  102  X  400 
cir.  mils  =  -  y  --  =  --  ^2  -  =  74,800  cir.  mils 

Referring  to  Table  154,  the  next  larger  wire  size  is  No.  1  which  has  an 
area  of  83,690  cir.  mils  and  safely  carries,  for  exposed  wire  (slow-burning 
or  weather-proof  insulation)  150  amp. 


148 


CENTRAL  STATIONS 


[ART.  188 


Since  to  conform  to  National  Electric  Code  rules  a  motor  branch  cir- 
cuit must  be  capable  of  carrying  at  least  25  per  cent,  over-current  the 
wires  to  this  motor  (Fig.  107)  must  be  capable  of  safely  carrying  102:  X 
1.25  =  127.5  amp.  The  No.  1  wire  will  do  this.  It  should  be  understood 
that  the  result,  74,800  cir.  mils,  obtained  above  is  not  exactly  accurate. 
With  102  amp.  flowing  and  the  load  at  80  per  cent,  power  factor  the  volts 
loss  in  the  line  (assuming  no  line  reactance)  will  be  something  less  than 
12  volts,  as  explained  in  the  following  article. 

188.  The  Actual  Volts  Loss  in  a  Single-phase  Alternating- 
current  Line  Where  It  Has  No  Reactance  Can  Be  Determined 
by  Drawing  a  Vector  Diagram  to  scale,  as  explained  in  the 
following  example.  This  method  can  be  used  for  the  solution 
of  any  problem  in  which  the  line  reactance  can  be  neglected. 


Scale  in  \Alts 

Fro.  108. — Voltage  diagram  of  circuit       FIG.    109. — Solution  of   problem  of 
with  no  line  reactance.  Fig.  107. 

DIRECTIONS. — See  Fig.  108.  Find  the  power  factor  of  the  load  or  use 
an  assumed  power  factor.  Refer  to  a  table  of  cosines*  and  find  the  lag 
angle  to  which  this  power  factor  corresponds.  Lay  out  the  angle  <f>  equal 
to  the  lag  angle.  Measure  off  distance  AB  proportional  to  the  voltage 
impressed  on  a  load.  Lay  off  EC,  parallel  to  AD,  proportional  to  the 
resistance  drop  in  the  line.  Then  AC  will  be  proportional  in  length  to 
the  voltage  impressed  on  the  line.  The  difference  in  length  between  AB 
and  BC  will  then  be  proportional  to  the  actual  volts  loss  in  the  line. 
This  difference  is  obtained  by  striking  the  arcs  EE1  and  FF1  and  measur- 
ing the  distance  between  them,  as  for  example,  GG1. 

EXAMPLE. — Fig.  109  shows  the  problem  of  Fig.  107  solved  graphically 
from  which  it  is  apparent  that  the  true  volts  loss  in  the  line  would  be  9 
volts,  whereas  the  formula  (54),  as  shown  in  preceding  Art.  183,  indicates 
that  the  loss  would  be  12  volts.  Actually,  12  volts  is  the  resistance  drop 

*  See  the  author's  AMERICAN  ELECTRICIANS'  HANDBOOK. 


SEC.  9]  ALTERNATING-CURRENT  CIRCUITS  149 

in  the  line.  It  may  seem  odd  that  the  total  volts  drop  in  the  line  is  less 
than  the  resistance  drop.  This  condition  is  due  to  certain  properties  of 
alternating  currents  and  frequently  occurs.  Instead  of  drawing  a  dia- 
gram like  that  of  Fig.  109,  the  problem  can  also  be  solved  by  using  the 
Mershon  diagram,  as  described  in  following  Art.  190A. 

189.  Where  It  Is  Necessary  to  Find  the  Resistance  Drop 
With  a  Given  Conductor  formula  (51)  may  be  utilized  as 
indicated  in  the  following  example. 

EXAMPLE.  —  What  is  the  resistance  drop  in  a  circuit  400  ft.  long  of  No. 
1  wire  when  it  is  carrying  102  amp.?  SOLUTION.  —  First  from  Table 
190A  it  is  found  that  the  area,  in  circular  mils,  of  a  No.  1  wire  is  83,690. 
Now  substituting  the  values  in  formula  (51): 

v       22  X  /  X  L       22  X  102  X  400       997,600 

air.  mtis  ~~      =  " 


190.  The  Graphic  Method  of  Computing  the  Wire  Size 
for  a  Single-phase  Alternating-current  Circuit  Where  the 
Line  Has  Reactance  will  now  be  explained.  There  is  no  direct 
method  of  solving  such  problems.  One  serviceable  method 
is  that  (which  will  be  explained)  of  assuming  a  certain  con- 
ductor on  the  basis  of  energy  (not  voltage)  loss  and  then 
checking  it  graphically  (or  with  the  Mershon  diagram  of  Fig. 
115)  to  ascertain  whether  or  not  the  voltage  loss  in  it  is  exces- 
sive. If  the  voltage  loss  is  excessive,  a  conductor  of  another 
size  must  be  tried  or  the  circuit  must  be  subdivided,  as  sug- 
gested in  a  preceding  article,  until  an  arrangement  of  con- 
ductors is  found  which  will  maintain  the  drop  within  the 
specified  limit.  The  graphic  method  is  best  explained  by 
the  solution  of  specific  examples.  Figs.  110  and  111  show 
typical  voltage  vector  diagrams  for  circuits,  the  component 
vectors  being  labeled  on  the  diagrams. 

EXAMPLE.  —  What  sizp  wire  should  be  used  for  the  single-phase  circuit 
of  Fig.  112?  The  load  consists  of  twelve  hundred  50-watt  incandescent 
lamps  (60,000  watts  total);  the  power  factor  is,  98  per  cent.;  the  feeder  is 
525  ft.  long  and  the  voltage  at  its  end  is  to  be  120  volts;  the  conductors 
are  supported  on  a  pole  line  8  in.  apart;  allowable  energy  loss  is  10  per 
cent,  of  the  energy  transmitted  and  the  voltage  drop  in  the  line  must  not 
exceed  10  or  12  per  cent.  (This  is  a  much  greater  voltage  drop  than  is 
ordinarily  allowable.  It  is  used  in  this  problem  to  exaggerate  the  values 


150 


CENTRAL  STATIONS 


[ART.  190 


Total  Drop* 
inline      ' 


^.-Reactance  Drop 
*  \ihLine 


onUne 


FIQ.  110. — Voltage  diagram  of  circuit  with  line  reactance,  load  at  less  than 
100  per  cent,  power  factor. 


E'      F' 


Fio.  111. — Voltage  diagram  of  circuit  with  line  reactance  and  load  at  100 
per  cent,  power  factor. 


,A.C.Gener«tor  60  Cycles 


Current  =  510. 2  Amp. 


S  60.000  '<* 


-525 ft.- 


PIG.  112. — Find  size  wire  for  circuit. 


SEC.  9] 


ALTERNATING-CURRENT  CIRCUITS 


151 


involved  and  thus  bring  out  the  facts.)     SOLUTION.  —  First  find  the  cur- 
rent which  will  flow  in  the  line,  thus: 

P  60,000 

7  =  VxjJ.  "  120X0.98 


510'2  amp- 


The  allowable  energy  loss  is  10  per  cent,  or  0.10  X  60,000  watts  =  6,000 
watts.  The  size  conductor  that  will  give  a  loss  of  6,000  watts  may  be 
determined  from  formula  (60)  thus: 


dr.  mils 


22  X  /*  X  L      22  X  510.2  X  510.2  X  525 


=  510,000  dr.  mils. 


Scale  in  Volts 
Fio.  113. — Solution  with  600,000-cir.  mil  conductor. 

That  is,  a  510,000-cir.  mil  conductor  would  maintain  the  energy  loss 
within  10  per  cent.  Consider  the  vector  diagram  of  Fig.  113.  Find  the 
lag  angle  <f>  corresponding  to  a  power  factor  of  98  per  cent.  This,  from 
a  table  of  cosines,  is  found  to  be  1 1  deg.  Lay  out  angle  <t>  equal  to  1 1  deg., 
as  indicated  in  Fig.  113.  Now,  lay  off  OB  proportional  in  length  to  the 
pressure  to  be  impressed  on  the  load,  120  volts. 

Lay  off  BD,  parallel  to  OA,  proportional  to  the  resistance  drop  in  the 
line.  This  resistance  drop  equals  the  line  resistance  times  the  line  cur- 
rent. From  Table  190A,  it  is  found  that  the  resistance  of  1,000  ft.  of 
600,000  cir.  mil  circuit  (2,000  ft.  of  line)  is  0.035  ohm.  Then  the  resist- 
ance drop  is  calculated  thus: 

7  X  R  =  510.2  X  0.035^  X  525 /*.  _  Q A  ^ 

That  is,  the  resistance  drop  in  the  line  of  Fig.  112  is  9.4  volts.  In  the 
diagram  of  Fig.  113  BD  is  then  laid  off  proportional  to  9.4  volts. 


152 


CENTRAL  STATIONS 


[ART.  190 


Now  lay  off  DC  at  right  angles  to  BD  proportional  to  the  reactance 
drop  of  the  line.  The  reactance  drop  equals  (the  line  reactance)  X  (the 
line  current).  From  Table  19(L4.  it  is  found  that  the  reactance  drop  of 
1,000  ft.  of  600,000-cir.  mil  circuit  (2,000  ft.  of  conductor)  is,  for  a  fre- 
quency of  60  cycles  and  a  wire  separation  of  8  in.,  0.144  ohm.  The  re- 
actance drop  is  now  calculated  thus: 


I  XX 


A  line,  OC  (Fig.  113),  is  then  drawn  and  it  will  be  proportional  to  the 
pressure,  137.5  volts  in  this  case,  which  must  be  impressed  on  the  end  of 
the  line  nearest  the  generator  (Fig.  112)  to  maintain  the  voltage  at  the 
load  at  120  volts. 

The  true  volts  line  loss  is  proportional  to  the  difference  between  the 
lengths  OB  and  OC.  It  is  obtained  by  striking  the  arcs  EE1  and  FF1 
and  measuring  the  distance  G  between  them.  In  this  example  the  true 


Scale  in  Volts 
FIG.  114. — Solution  with  300,000-cir.  mil  conductors. 

line  loss  is  20  volts.  The  percentage  volts  drop  is  20  -f-  120  =  16.6  per 
cent.,  which  is  too  high  to  satisfy  the  requirements  of  this  example, 
wherein  it  is  specified  that  "the  allowable  voltage  drop  in  the  line  must 
not  exceed  10  or  12  per  cent."  Hence,  another  conductor  arrangement 
must  be  tried. 

The  volts  line  drop  in  this  case  is  due  mainly  to  reactance  (Fig.  113) 
and  can,  therefore,  be  decreased  by  decreasing  the  reactance.  The  re- 
actance could  be  made  smaller  by  bringing  the  wires  closer  together  but 
the  example  (Fig.  112)  specifies  that  conductors  are  to  be  8  in.  apart. 
The  other  method  of  decreasing  the  reactance  drop  is  to  subdivide  the 
circuit.  This  will  now  be  tried.  Two  circuits  of  300,000  cir.  mils  con- 
ductors in  parallel  will  be  considered  instead  of  one  600,000-cir.-mil  cir- 


SEC.  9] 


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CENTRAL  STATIONS 


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as  to  its  use  and  application. 


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155 


156  CENTRAL  STATIONS  [ART.  191 

cuit.  With  the  two  300,000-cir.  mil  circuits,  the  resistance  drop  will 
remain  the  same  as  before,  because,  although  each  conductor  is  one-half 
the  size  it  was  formerly,  it  carries  but  one-half  the  current  that  it  did 
formerly. 

Lay  out  a  diagram  like  that  of  Fig.  114.  The  angle  </>  will  again  be  11 
deg.  and  the  lines  OB  and  BD,  representing,  respectively,  the  voltage 
impressed  on  the  load  and  the  resistance  drop  in  the  line,  will  be  propor- 
tional to  120  and  to  9.4  volts  as  before. 

To  lay  out  CD,  which  is  proportional  in  length  to  the  reactance  drop 
in  the  line,  refer  to  Table  190A  and  note  that  the  reactance  of  2,000  ft. 
of  300,000  cir.  mil  conductor  (1,000  ft.  of  circuit)  is,  for  an  8-in.  separa- 
tion and  a  frequency  of  60  cycles,  0.160  ohm.  Then  the  reactance 
drop  will  be  (the  current  is  now  one-half  of  the  former  current  or  510.2  -f- 
2  =  255.1  amp.): 


7  X  X  =  (255.1  amp.)  X  =  21.4  volts. 


DC  (Fig.  114)  is,  then,  laid  off  proportional  in  length  to  21.4  volts. 
The  line  OC  is  now  drawn  and  it  is  found  that  it  scales  134  volts.  The 
true  volts  drop  in  the  line,  G,  is  found  to  measure  13.5  volts.  The  per- 
centage drop  is,  therefore,  now  13.5  -r-  120  =  11.3  per  cent.  Hence,  the 
arrangement  obtained  by  subdividing  the  circuit  into  two  300,000-cir. 
mil  circuits  in  parallel  meets  the  requirements  of  the  example. 

191.  The  Determination  With  a  Mershon  Diagram  of  the 
Wire  Size  for  a  Single-phase  Alternating-current  Circuit 
Where  the  Line  Has  Reactance  will  now  be  considered.  The 
problem  solved  in  preceding  Art.  190  and  similar  ones  can  be 
handled  more  rapidly  and  with  less  effort  by  using  the  Mershon 
diagram  (Fig.  115)  than  by  following  the  graphic  method  just 
outlined  in  Art.  190.  However,  it  is  desirable  that  one  be 
familiar  with  the  graphical  method  of  Figs.  112  and  113,  be- 
cause it  can  be  utilized  when  the  Mershon  diagram  graph 
(Fig.  115)  is  not  available.  Furthermore,  if  one  understands 
the  method  of  Figs.  112  and  113  he  will  more  readily  compre- 
hend the  application  of  the  Mershon  diagram. 

NOTE.  —  In  using  the  Mershon  diagram,  the  general  procedure  is  prac- 
tically the  same  as  with  the  graphical  method.  The  conductor  size  is 
first  tentatively  computed  by  applying  a  formula  so  that  the  energy  loss 
will  be  a  certain  percentage  of  the  energy  transmitted.  The  tentative 
conductor  size  thus  obtained  is  checked  with  a  Mershon  diagram  to  as- 
certain whether  or  not  the  true  volts  line  drop  which  will  occur  with  it 
will  be  excessive.  If  the  line  drop  with  a  conductor  of  this  size  is  exces- 


SEC.  9]  ALTERNATING-CURRENT  CIRCUITS  157 

sive  then  a  different  size  conductor  or  a  different  arrangement  of  conduc- 
tors must  be  used. 

EXAMPLE. — What  size  wire  should  be  used  for  the  feeder  circuit  of 
Fig.  116?  The  load  consists  of  24  kw.  (24,000  watts)  of  mercury  vapor 
lamps.  The  circuit  is  100  ft.  long.  The  pressure  at  the  receiver  end 
should  be  240  volts.  The  wires  are  carried  in  conduit.  The  power  fac- 
tor of  the  mercury  vapor  lamp  load  is  98  per  cent.  The  frequency  is 
60  cycles.  The  energy  loss  should  not  exceed  2  per  cent.  The  true  volt 
line  drop  should  not  exceed  2  per  cent,  of  the  receiver  voltage.  SOLUTION. 
—The  allowable  energy  loss  =  0.02  =  24,000  X  480  watts.  Allowable 
volts  drop  =  0.02  X  240  =  4.8  volts. 


Switch                                            ''-Wrought  Iron  Should  be-' 

---60-Cyde,  Shale-Phase.  A.  C.  Bus  Bars              Conduit  240  Volts  Here 
(Source  of  Electrical  Energy) 

FIG.  116.  —  Determination  of  size  of  wire  for  an  alternating-current  circuit 
in  conduit. 

P                 24,000  100 

Line  current  =  I  =                   -  240  X  0.98  =  O98  =  102  °mp" 


To  compute  the  size  conductor  that  will  give  a  480-watt  energy  loss 
formula  (60)  is  used,  thus  : 

72  X  22  X  L       102  X  102  X  22  X  100 

cir.  mils    =  —  —  p  —  —  2^n  —  ~~   =  47,700  CM.  mus. 

Now  a  47,700-cir.  mil  conductor  (Table  190A)  most  nearly  corresponds 
to  a  No.  3  wire  which  has  an  actual  area  of  52,630  cir.  mils  and,  with 
rubber  insulation  (which  must  be  used  in  conduit  wiring)  will  safely 
carry  76  amp.  However,  the  load  in  this  problem  is  102  amp.,  so  No.  3 
can  not  be  used.  No.  1  wire,  which  (Table  190A)  safely  carries  100  amp. 
and  which  would  be  safe  for  the  102  amp.  of  this  problem,  will  be  checked 
for  true  volts  drop  by  using  the  Mershon  diagram.  To  use  the  diagram 
it  is  first  necessary  to  find  the  resistance  drop  and  the  reactance  drop. 
From  Table  190^4,  the  resistance  of  1,000  ft.  of  No.  1  two-wire  circuit 
(2,000  ft.  of  wire)  is  0.248  ohms.  Then  the  resistance  drop,  which  equals 
current  multiplied  by  resistance,  of  the  100  ft.  of  circuit  of  this  problem 


/  X  R  =  (102)  X  -QQQ        =  2.5  vote. 

Then  the  percentage  voltage  drop  is:  2.5  •*•  240  =  1.04  per  cent. 
Also  from  Table  190A,  the  reactance  of  1,000  ft.  of  No.  1  wire  circuit  for 
60  cycles  and  a  J^-in.  separation  between  conductors  (No.  1  conductors 


158 


CENTRAL  STATIONS 


[ART.  191 


in  conduit  are  about  }•£  in.  between  centers)  is  0.028  ohm.  Then  the 
reactance  drop,  which  equals  current  times  reactance  is,  for  100  circuit  ft. : 
0.028  X  100 


7  X  X  =  (102)  X 


1,000 


=  0.286  volts. 


The  percentage  drop  is:  0.286  •«•  240  =  0.00119  =  0.12  per  cent. 
Now  refer  to  the  Mershon  diagram  of  Fig.  115  and  lay  off  the  percentage 
resistance  and  reactance  drops  above  found  as  suggested  in  Fig.  117. 


FIG.  117. — Showing  the  application  of  the  Mershon  diagram  to  the  problem 
of  Art.  191. 

Find  the  vertical  line  in  the  diagram  corresponding  to  the  power  factor — 
98  per  cent,  in  this  example — of  the  load  and  follow  it,  AB,  upward 
until  it  intersects  the  smallest  circle  marked  0.  From  this  point  of 
intersection  lay  off  to  the  right  horizontally  the  percentage  resistance 
drop,  that  is,  1.04  (about  1  horizontal  division  in  length)  and  from  this 
last  point  lay  off  vertically  upward  the  percentage  reactance  drop,  0.29 
(about  Ho  of  1  vertical  division).  This  last  point  lies  just  inside  of  the 
1  per  cent,  circle  so  the  true  volts  loss  with  a  No.  1  wire  would  be  about 
1  per  cent.  That  is,  the  drop  would  be  0.01  X  240  volts  =  2.4  volts. 


SEC.  9]  ALTERNATING-CURRENT  CIRCUITS  159 

The  voltage  impressed  on  the  end  of  the  circuit  nearest  the  generator 
would  have  to  be:  240  +  2.4  =  242.4  volts.  The  true  line  drop,  2.4 
volts,  is  well  within  the  4.8-volt  limit  specified  in  the  example  and  the 
energy  loss  will  be  less  than  2  per  cent,  because  it  was  necessary  to  use 
No.  1  wire  to  carry  the  current. 

EXAMPLE.  —  What  size  rubber-insulation  wire  should  be  used  for  the 
circuit  (Fig.  118)  to  the  50-h.p.,  60-cycle,  single-phase  induction  motor 
there  illustrated?  The  efficiency  of  the  motor  is  90  per  cent.  Its  power 
factor  is  85  per  cent.  The  conductors  are  to  be  exposed  and  4  in.  apart. 
A  4  per  cent,  energy  loss  is  allowable  and  the  true  volts  line  drop  must 
not  exceed  6  or  7  per  cent.  SOLUTION.  —  First  find  the  load  in  apparent 
watts: 

Apparent  watts  =        '™         *         .  =  48,758  apparent  watts. 


apparent  watts       48.758 
Line  current  =  —   —  —   —  =  =  200  amp. 


Source  of  Energy  SO  hp,  60-Cucle.  240  Volt.  Single: 

(A.C.6enerator)  Phase  Induction  Motor 


-  1       4  'Separation  Between  Conductors 

------------  .......  „  ........  :..600  Ft.~~-  .........  -  ....................  Jj 

Motor  Efficiency  is  90%  ; 
Power  Factor  is  65% 

FIG.  118.  —  Another  example  in  computing  size  of  an  alternating-current 
circuit  conductor. 

(Usually  in  solving  practical  motor-circuit  examples  the  current  can 
be  read  directly  from  tables.) 

Actual  watts  =  apparent  watts  X  p.f.  =  48,758  X  0.85  =  41,500  watts. 
Allowable  energy  loss  is  4  per  cent.  =  0.04  X  41,500  =  1,660  watts. 

The  size  conductor  that  will  give  a  1,660-watt  line  loss  with  a  line  cur- 
rent of  195  amp.  is  found  thus: 

„,  ma,  ,  JlxfxL  ,  aooxaooxMoxa  _  mjmjm  _ 

318,000  dr.  mils. 

Try  a  300,000-cir.  mil  conductor  which  safely  carries  275  amp.  Find 
the  resistance  and  reactance  drops  in  the  line  in  the  same  way  as  in  the 
preceding  example,  taking  values  for  resistance  and  reactance  of  the 
300,000-cir.  mil  cable  from  Table  190A. 


Then  the  percentage  resistance  drop  =  9.0  -*•  240  =  3.75  per.  cent. 
I  X  X  =  (200)  X  °'13140^)600  -  16.1  volts. 


160 


CENTRAL  STATION'S 


[ART.  192 


Then  the  percentage  reactance  drop  =  16.1  -5-  240  =  6.7  per  cent. 

Now  lay  off  this  percentage  resistance  and  reactance  drop  on  the  Mer- 
shon  diagram  of  Fig.  115  (as  shown  in  the  enlarged  view  of  Fig.  119)  at 
the  point,  P,  corresponding  to  85  per  cent,  power  factor,  in  the  same  man- 


0.80  .0.90  0  5 

FIG.  119. — Mershon  diagram  solution  of  the  problem  of  Fig.   115. 

ner  as  in  the  above  example.  The  true  volts  drop  is  found  to  be  6  per 
cent.  Or,  in  actual  volts,  the  true  line  drop  is:  0.06  X  250  =  15  volts. 
This  satisfies  the  requirements  of  the  example. 

192.  The  Determination  of  the  Wire  Size  for  Two-phase, 
Alternating-current  Circuits  may  be  made  on  this  basis :  A 
four-wire,  two-phase  circuit  may,  so  far  as  energy  loss  and  voltage 


SEC.  9]  ALTERNATING-CURRENT  CIRCUITS  161 

reactance  drop  are  concerned,  be  replaced  by  two  single-phase 
circuits  identical  (as  to  size  of  wire,  distance  between  wires, 
current  and  e.m.f.}  with  two  circuits  of  the  two-phase  transmis- 
sion, provided  that  in  both  cases  there  is  no  inductive  interaction 
between  circuits.  Therefore,  to  calculate  a  four-wire,  two-phase 
circuit,  compute  the  single-phase  circuit  required  to  transmit 
one-half  the  power  at  the  same  voltage.  Then  the  two-phase 
transmission  will  require  two  such  circuits. 

193.  The  Determination  of  the  Wire  Size  for  a  Two-phase, 
Alternating-current  Circuit  Where  the  Line  Reactance  Is 
Negligible  may  be  based  on  the  truth  outlined  in  the  preced- 
ing Art.  192.  This  method  may,  ordinarily,  be  used  for 
interior  wiring  circuits  and  under  the  same  conditions  as 
specified  in  preceding  articles  for  single-phase  circuits.  If  the 
power-factor  of  the  load  is  100  per  cent.,  the  load  balanced, 
and  the  line  has  no  reactance  the  result  obtained  by  using 
the  equation  given  below  will  (assuming  11  ohms  is  the  resist- 
ance of  a  circuit  mil-foot  of  copper)  be  correct.  If  the  power 
factor  of  the  load  is  less  than  100  per  cent,  and  the  line  has 
no  or  very  little  reactance  the  true  volts  drop  in  the  line  will, 
as  outlined  in  Art.  188,  be  something  less  than  the  volts  drop 
represented  by  V  in  the  following  formulas. 

NOTE.  —  In  calculating  a  two-phase  circuit  by  the  method  to  be  de- 
scribed, the  first  step  (unless  the  current  per  phase  is  known)  is  to  find 
one-half  of  the  total  power  load  fed  by  the  circuit.  Then  find  the  cur- 
rent in  amperes  corresponding  to  this  one-half  total  power  load  with  a 
balanced  two-phase  four-wire  circuit.  The  current  corresponding  to 
one-half  the  total  load  will  be  the  current  in  the  outside  wires,  hence, 
may  be  computed  with  the  following  formula: 

(61)  /  =  0.50  X  (amperes) 


Wherein.  —  7  =  the  line  current,  in  amp.  P  —  the  actual  power  load  in 
watts.  E  =  the  voltage  impressed  on  the  load.  p.f.  =  the  power  factor 
of  the  load. 

When  the  current  value,  7,  has  been  obtained  with  the  above  formula 
or  if  the  current  per  phase,  which  is  the  same  thing,  is  known  it  is  substi- 
tuted in  formula  (54)  which  is: 

22X1  XL 
ctr.  mils  «=  -  ^  -  * 


162 


CENTRAL  STATIONS 


[ART.  193 


Wherein.— Cir.  mils  «»  area  of  conductors,  four  of  which  will  be  required. 
/  =  current  in  each  phase,  in  amperes.  L  =»  single  distance  of  the  cir- 
cuit, in  feet.  V  =  allowable  volts  drop  in  the  circuit. 

EXAMPLE. — What  size  wire  should  be  used  for  the  two-phase  circuit 
of  Fig.  120?  The  load  consists  of  55  kw.  in  incandescent  lamps.  The 
power  factor  is  100  per  cent.  The  single  distance  is  300  ft.  The  allow- 
able drop  is  2  volts.  Conductors  are  to  be  carried  in  conduit,  hence  line 
reactance  will  be  small  and  can  be  neglected. 

SOLUTION. — First  find  the  line  current  (current  per  phase  from  formula 
(61)  above)  thus: 


.-Practically  no  Reactance 


300 Ft.  ?  Volts  Drop--- 


<•  •  Two-Phase  Mains 

FIG.  120. — Size  wire  for  two-phase  circuit. 


55  X  1,000 
110  X  1.0 


0.50  X 


55,000 
110 


250  amp. 


Therefore,  250  amp.  will  flow  in  each  wire.     Now  substitute  in  the  for- 
mula (54) : 

,        22  X  /  X  L      22X250X300       0_c  nAn    . 
cir.  mils  =  — — v—  =  —  —  =  875,000  cir.  mils. 


4- 400.000  Cir.  Mil 
Switches  ..-•  fuses         ..-.Conductors  110  \ 


FIG.  121. — Arrangement  of  conductors  for  example  of  Fig.   120. 

A  900,000-cir.  mil  cable  might  be  used  which  would  (from  Table  190A) 
safely  carry  600  amp.  Four  such  cables  would  be  required  for  the  cir- 
cuit. But  a  900,000-cir.  mil  cable  would  be  altogether  too  large  to 
handle  readily  to  draw  into  conduit.  Furthermore,  its  skin  effect* 
would  be  excessively  large.  Hence,  the  circuit  should  be  split  up  and 
arranged  into,  possibly,  one  sub-circuit  of  four  400,000-cir.  mil  conductors 
and  one  sub-circuit  of  four  500,000-cir.  mil  conductors  as  suggested  in 
Fig.  121.  Cables  of  these  sizes  can  be  handled  readily  and  their  skin 
effects  would  be  relatively  small,  particularly  if  they  were  made  with 
fibre  cores.*  Checking  with  a  table  of  safe-current-carrying  capacities, 
it  is  evident  that  these  conductors  would  be  amply  large  to  carry  the 
current. 

•  See  the  author's  PRACTICAL  ELECTRICITY  and  AMERICAN  ELECTRICIANS'  HANDBOOK. 


SEC.  9]  ALTERNATING-CURRENT  CIRCUITS  163 

EXAMPLE.— What  size  wire  should  be  used  for  the  branch  circuit  to 
the  220-volt  two-phase  motor  of  Fig.  122?  The  motor  is  rated  on  its 
nameplate  as  taking  44  amp.  per  phase  (if  the  ampere  per  phase  is  not 
given  it  can  be  computed  from  the  formula  given  in  the  preceding  ex- 
ample). The  circuit  is  110  ft.  long.  The  allowable  drop  is  3  per  cent. 
SOLUTION—  The  allowable  volts  drop  is:  0.03  X  220  =  6.6  or  say  7 
volts.  Now  substitute  in  formula  (54) : 

22  X  /  X  L      22  X  44  X  110      106,480 
Cir.  mils  = y =  -     — ^ = ~ =  15,211  dr.  mils. 

<    110  Ft.  7  Volts 


FIG.  122. — Find  size  wire. 

From  Table  190 A  the  next  larger  standard  wire  size  is  No.  8  which  has 
an  area  of  16,510  cir.  mils  and  which  will,  with  rubber  insulation,  safely 
carry  35  amp.  This  being  a  branch  circuit  to  a  motor,  it  must  be  capable 
of  safely  carrying  at  least  a  25  per  cent,  overload:  1.25  X  44  =  55  amp. 
Hence,  No.  4  wire,  the  smallest  size  which  will  safely  carry  55  amp.,  must 
be  used.  Four  No.  4  wires  from  the  switch  to  the  motor  would  constitute 
the  circuit. 

194.  The  Determination  by  the  Graphic  Method  of  the  Wire 
Size  for  a  Two-phase  Circuit  Where  the  Line  Has  Reactance 

may  be  made  on  the  following  basis.     The  computations  are 


•2 -Phase 

C-entrator.  WCycles 


FIG.  123. — Find  size  wire  for  two-phase  circuit. 

based  on  the  following  methods  which  are  similar  to  those  for 
single-phase  circuits  where  the  line  has  reactance.  They  may, 
therefore,  be  made  either  graphically  or  with  the  Mershon  dia- 
gram of  Fig.  115.  The  first  step  is  to  find  one-half  the  load 
on  the  circuit  under  consideration.  Then  proceed  with  the 
graphical  or  the  Mershon  diagram  solution  just  as  if  the  cir- 
cuit were  a  single-phase  circuit  carrying  this  one-half  load. 

EXAMPLE.— What  size  wire  should  be  used  for  the  feeder  of  Fig.  123? 
It  serves  a  two-phase  load  comprising  twenty-four  hundred  50-watt  in- 


164  CENTRAL  STATIONS  [ART.  195 

candescent  lamps.  The  power  factor  is  98  per  cent.  The  circuit  is  500 
ft.  long.  The  pressure  at  the  load  end  of  the  feeder  should  be  120  volts. 
The  conductors  are  supported,  8  in.  between  centers,  on  a  pole  line.  The 
allowable  energy  loss  is  10  per  cent,  of  the  energy  transmitted.  The  true 
voltage  drop  in  the  line  must  not  exceed  10  or  12  per  cent.  SOLUTION. — 
Find  one-half  of  the  total  load  thus:  2,400  watts  X  50  waits  per  lamp  = 
120,000  watts  is  the  total  load.  Now  one-half  the  total  load  is :  (120,000 
watts)  +2  =  60,000  watts. 

From  this  point  on  the  example  is  solved  by  precisely  the  same  method 
as  that  illustrated  in  connection  with  Fig.  112  (the  load  in  the  present 
example  was  taken  purposely  just  twice  that  of  the  Fig.  112  load  to 
illustrate  the  principle).  For  this  two-phase  circuit  with  a  120,000-watt 
total  load  four  600,000-cir.  mil  conductors  might  be  used  and  with  them 
the  true  volts  line  drop  would  be  21  volts  (Fig.  113)  or  the  same  as  if 
two  600,000-cir.  mil  conductors  were  used  with  a  60,000-watt  load  on 
a  single-phase  circuit. 

Since,  however,  a  12  per  cent,  drop  should  not  exist  in  the  circuit  of 
this  problem,  each  600,000-cir.  mil  conductor  can  be  split  into  two 
300,000-cir.  mil  conductors  in  order  to  reduce  the  line  reactance.  With 
the  conductors  thus  split  up,  as  in  the  example  of  Fig.  121,  the  true  volts 
line  loss  would  be  (Fig.  114)  14.5  volts  or  12  per  cent.,  which  meets  the 
conditions  of  this  example.  Eight  300,000-cir.  mil  conductors  would 
then  be  required  for  this  two-phase  transmission  and  they  should  be 
arranged  in  a  manner  similar  to  that  suggested  in  the  example  of  Fig.  121. 


FIG.  124. — Find  size  wire  and  voltage  drop. 

195.  The  Determination,  With  the  Mershon  Diagram,  of 
the  Wire  Size  for  a  Two-phase  Circuit  Where  the  Line  Has 
Reactance  will  now  be  explained.  As  with  the  graphic  method 
described  in  the  preceding  article,  the  first  step  is  to  find  one- 
half  of  the  total  load  on  the  circuit.  Then  proceed  with  this 
one-half  total  load  as  if  it  were  the  entire  load  on  a  single- 
phase  circuit. 

EXAMPLE. — What  size  wire  should  be  used  for  the  two-phase  feeder 
circuit  of  Fig.  124?  The  load  consists  of  48  kw.  (48,000  watts)  in  quartz 
lamps.  The  circuit  is  100  ft.  long.  The  pressure  at  the  receiver  end  of 
the  circuit  is  to  be  240  volts.  The  wires  are  carried  in  conduit.  The 
power  factor  of  the  lamp  load  is  98  per  cent.  The  frequency  is  60  cycles. 


SEC.  9]  ALTERNATING-CURRENT  CIRCUITS  165 

The  energy  loss  should  not  exceed  2  per  cent.  The  volts  line  drop  should 
not  exceed  2  per  cent,  of  the  receiver  voltage.  SOLUTION. — First  find 
one-half  the  total  load,  thus:  (48,000)  4-  2  =  24,000  watts.  From  this 
point  on  the  example  is  solved  in  precisely  the  same  manner  as  that  of 
Figs.  116  and  117.  (The  load  in  the  present  example  was  taken  purposely 
just  twice  that  in  the  single-phase  example  of  Fig.  116  in  order  to  illus- 
trate the  principle.)  The  other  conditions  of  the  present  two-phase  ex- 
ample are  the  same  as  those  of  the  single-phase  example  of  Fig.  116. 

For  this  two-phase  circuit  with  a  48,000-watt  total  load  four  No.  1 
wires  should  be  used  and  with  them  the  true-volt  line  loss  will  be  about 
1  per  cent,  or  2.4  volts — the  same  as  in  the  two  conductors  of  the  single- 
phase  circuit  serving  the  24,000-watt  load  in  Fig.  116. 


<-~2-Pha$e.  60  Cycle  Supply 


•'250 V.     •«  OS% 
2  -Phase  Motor  I00h.p.\  Power 


FIG.  125. — Find  size  wire  and  voltage  drop. 

EXAMPLE. — The  result  for  the  example  of  Fig.  125,  which  shows  a 
100-h.p.  motor  fed  by  a  two-phase  circuit,  is  the  same  as  that  for  the  ex- 
ample of  Fig.  116  which  shows  a  50-h.p.  motor  fed  by  a  single-phase  cir- 
cuit. All  of  the  conditions,  with  the  exception  of  the  horse-power  rating 
of  the  motor,  are  the  same  for  both  problems.  The  problem  is  worked 
out  for  Fig.  118  for  the  single-phase  50-h.p.  load.  The  solution  for  the 
two-phase  circuit  with  a  100-h.p.  load  (twice  the  single-phase  load)  is 
precisely  the  same  as  for  the  50-h.p.  single-phase  load,  after  one-half  of 
the  two-phase  load  has  been  found  thus:  (100  h.p.}  -r-  2  =  50  h.p. 
However,  two  300,000-cir.  mil  conductors  are  used  for  the  50-h.p.  single- 
phase  circuit  and  four  400,000-cir.  mil  conductors  are  used  for  the  100- 
h.p.  two-phase  circuit. 

196.  The  Determination  of  the  Wire  Size  for  a  Three-phase 
Circuit  Where  the  Line  Reactance  Is  Small  and  may,  there- 
fore, be  disregarded,  may  be  made  with  the  following  formula, 
the  derivation  for  which  is  given  in  the  following  note: 

1 Q  v   T  V  T 

(62)  dr.  mils  =  -  ^jr--  (circular  mils) 

(63)  V  =  19  X  J  X  L  (volts) 

cir.  mils 

/QJ|N  T       V  X  dr.  mils 

(64)  I  =  — 1ft  ^  _  (amperes) 


166 

(65) 


CENTRAL  STATIONS 


L  = 


V  X  dr.  mils 
19  XI 


[ABT.  197 
(feet) 


Wherein:  Cir.  mils  =  area,  in  circular  mils,  of  each  of  the  three 
wires  of  the  balanced  three-phase  circuit.  /  =  the  current,  in 
amperes,  in  each  of  the  three  wires.  L  =  the  single  distance  or 
length  one  way  of  the  circuit  in  feet.  V  =  the  allowable 
drop,  in  volts,  in  the  line. 

197.  The  Conditions  Under  Which  the  Above  Three-phase 
Formulas  May  be  Used  can 
be  specified  thus:  they  are 
ordinarily  sufficiently  accurate 
for  interior- wiring  circuits  and, 
under  the  conditions  as  speci- 
fied in  Art.  186,  for  single- 
phase  circuits  of  small  line  re- 
actance. If  the  power  factor 
of  the  load  is  100  per  cent., 
the  load  balanced  and  the  line 
has  no  reactance,  the  result 
given  by  the  preceding  form- 
ulas will  be  theoretically  cor- 
rect, assuming  the  resistance  of  a  circular  mil-foot  of  cop- 
per is  11  ohms.  If  the  power  factor  of  the  load  is  less 
than  100  per  cent,  and  the  line  has  no  or  very  little  reactance 
the  true  volts  drop  in  the  line  will  be  something  less  than  the 
volts  drop  represented  by  V  in  the  preceding  equations;  see 
Fig.  108  for  an  illustration  of  the  principle  as  applied  to  single- 
phase  circuit. 

NOTE. — The  Derivation  of  the  Above  Formulas  for  Determining  Three- 
phase  Wire  Sizes  Where  Line  Reactance  May  Be  Disregarded. — The  voltage 
relations  in  a  three-phase  circuit  may  be  represented  by  the  diagram 
shown  in  Fig.  126.  The  values  in  this  particular  diagram  apply  to  the 
circuit  diagrammed  in  Fig.  127.  This  diagram  (Fig.  127)  shows  how 
voltmeters  would  read  if  connected  to  the  three-phase  circuit  there  dia- 
grammed. The  voltage  impressed  on  the  circuit  is  1 10  volts.  The  drop 
in  each  of  the  three  line  wires,  AB,  AlBl  and  AUB11  is  1.16  volts.  From 
this  it  might  be  assumed  that  the  voltage  impressed  on  the  load  would 
be:  110  -  1.16  =  108.84  volts;  however,  such  is  not  the  case.  Tlv>  volt- 


Fio.  126. — Showing  vector  relations 
of  drops  in  a  balanced  three-phase 
circuit. 


SEC.  9]  ALTERNATING-CURRENT  CIRCUITS  167 

age  between  wires  at  the  load  would  actually  be,  as  shown  in  the  picture, 
108  volts. 

The  reason  for  this  is  that  the  e.m.fs.  in  AB,  AlBl  and  A"B»  are  not 
in  phase  with  one  another.  Hence,  the  voltage  drops  in  these  three 
conductors  are  not  in  phase  with  one  another.  Since  the  e.m.fs.  in  the 
three  wires  of  the  three-phase  circuit  differ  in  phase  by  120  deg.  if  there 
is  a  drop  of  1.16  volts  in  each  of  the  conductors  the  total  drop  across  any 
two  of  the  conductors  will  be  (as  shown  in  Fig.  126)  1.16  X  1.73  =  2 
volts. 

Now,  the  drop  in  one  of  the  wires  of  a  balanced  three-phase  circuit 
(for  example  AB,  Fig.  127)  may  be  computed  from  formula  (46): 


FIQ.  127.  —  Illustrating  drop  in  voltage  in  a  three-phase  circuit. 

V  =  11  X/XL 
ctr.  mils 

The  drop  obtained  by  the  above  equation  is  sometimes  called  "the  drop 
to  neutral."  Now  it  is  evident  from  Fig.  126  that  the  drop  in  any  two 
of  the  conductors  of  a  three-phase  circuit,  for  example  in  AB  and 
AlBl  (Fig.  127)  would  be:  1.73  X  the  drop  in  one  of  the  conductors. 
Thus  the  total  drop  in  the  two  conductors  may  be  expressed  thus: 

v  =  11  X  7  X  L  X  1.73  =  19  X  /  X  L 
dr.  mils  dr.  mils 

Hence,  from  the  above  equation  it  follows  that  for  a  three-phase  circuit: 


EXAMPLE.  —  What  size  wire  should  be  used  for  the  circuit  to  the  three- 
phase  220-volt  motor  of  Fig.  128?  The  circuit  is  400  ft.  long.  It  is 
carried  in  conduit  so  that  the  line  reactance  is  negligible.  The  allowable 
drop  is  6  volts.  The  motor  takes  40  kw.  It  is  assumed  that  its  power 
factor  is  but  70  per  cent.  First,  find  the  current  thus  (for  the  derivation 
of  the  following  formula,  see  the  author's  AMERICAN  ELECTRICIANS' 
HANDBOOK  or  his  PRACTICAL  ELECTRICITY)  : 


168 


CENTRAL  STATIONS 


[ART.  197 


kw.  X580       40  X580 


"    E  X  p./.    ~  220  X  0.7 
Now  substitute  in  the  above  formula  (62) : 

19  X  /  X  L       19  X  151  X  400 
cir.  mils  = ^ =  ~ — 


amp' 


=  191,266  dr.  mils. 


•f-  -  -Three -Phase  Supply  Main 


Three- Phase,  ?20-Vol1--' 
Induction  Motor 


FIG.  128. — Find  wire  size  for  three-phase  circuits. 

Referring  to  Table  190A;  the  next  larger  wire  size  is  No.  000  which  has 
an  area  of  211,600  cir.  mils.  Since  this  is  a  motor  circuit,  it  must  be 
capable  of  carrying  a  current  overload  of  at  least  25  per  cent.,  hence  this 
circuit  must  be  capable  of  safely  handling:  151  X  1.25  =  188.8  amp. 
Now  No.  0000  will  safely  carry  225  amp.  with  rubber  insulation  or  325 
with  other  insulations,  which  is  satisfactory  as  a  conductor  for  this 
example. 


n 

Total  Load  -1.000  Watts 

-o 

-o- 

> 

SOO  Watts  Load-;.'. 

~o~ 

-  j 

on  Each  Branch  J 

j 

~o~ 

> 

..Fuses 
j,       p-  Switches                                                                Fuses  ---y 

j 

— 

^^                                                                               i       ^,/IOK 

""-""•->      3                                                                             //0tf    ^ 

j                                                                            /i/s*s-J* 

> 

M 

> 

^    Three-Phase                                                         SOO  Watts  Lo«d:;; 
*'  Bus  -Bars                                                              on  Each  Branch-^ 

-:;>- 

-^ 

->• 

u 

Power  Factor  =  98% 

>> 

<H 

-^ 

FIG.  129. — Find  wire  size  for  three-phase  circuit. 

EXAMPLE. — What  size  conductor  should  be  used  for  the  three-phase, 
110-volt  feeder  circuit  of  Fig.  129?  The  load  consists  of  3  kw.  (3,000 
watts)  of  incandescent  lamps  at  a  power  factor  of  98  per  cent.  The 
wires  are  strung  close  together  so  that  the  line  reactance  can  be  neglected. 
The  circuit  is  400  ft.  long  and  the  allowable  drop  is  3  volts.  SOLUTION. — 
First  find  the  current  which  would  flow  in  this  circuit. 


I  = 


kw.  X  580  _     3  X  580 
E  X  p.f.   ~  110  X0.98 


16.2  amp. 


Now  substitute  the  above  current  value  in  formula  (62)  thus: 

•j        19  X  /  X  L       19  X  16.2  X  400        .,  .....    .        .. 

cir.  mils  = ^ =  —     - — - —     —  =  41,040  ctr.  mils. 


SEC.  9]  ALTERNATING-CURRENT  CIRCUITS  169 

The  next  larger  wire  size  (referring  to  Table  1904)  is  No.  4,  which  has 
an  area  of  41,740  cir.  mils.  This  conductor  will  more  than  safely  carry 
the  16.2  amp.  of  this  example,  hence  is  safe  and  may  be  used. 

198.  A  Three-wire  Three-phase  Transmission  May  Be 
Replaced  by  Two  Single-phase  Circuits. — It  is  frequently  de- 
sirable to  utilize  this  fact  in  making  three-phase  circuit  wiring 
calculations,  particularly  where  the  three-phase  circuit  has 
reactance.     That  is,   a  three-wire  three-phase  transmission 
having  conductors  symmetrically  located  may,  so  far  as  energy 
loss  and  voltage  requirements  are  concerned,  be  replaced  by 
two  single-phase  circuits  having  no  inductive  interaction  and 
identical  with  a  three-phase  line  as  to  size  of  wire  and  distance 
between  wires.     Therefore,  to  calculate  a  three-phase  trans- 
mission calculate  a  single-phase  circuit  to  carry  one-half  the 
load  at  the  same  voltage.     The  three-phase  transmission  will 
require  three  wires  of  the  size  and  distance  between  centers  as 
obtained  with  the  single-phase  transmission. 

199.  The  Determination  of  the  Wire  Size  for  a  Three-phase 
Circuit  Where  the  Line  Has  Reactance  may  be  made  either 
graphically  or  with  the  Mershon  diagram  (Fig.  115).     Where 
either  the  Mershon  or  the  graphical  method  is  used  the  fact 
outlined  in  Art.  198,  that  a  three-phase  circuit  can  be  replaced 
by  two  single-phase  circuits,"  is  utilized.     Regardless  of  whether 
the  Mershon  or  graphical  method  is  used,  first  find  one-half 
of  the  total  load.     Then  proceed  with  the  problem  using  this 
one-half  total  load  just  as  if  it  were  fed  by  one  single-phase 
circuit.     The  method  of  solving  three-phase  problems  is  simi- 
lar to  that  used  for  the  two-phase  examples  above  given, 
except  that  three  wires  of  the  size  obtained  are  used  for  three- 
phase  circuits,  whereas  four  wires  are  used  for  the  two-phase 
circuits. 

EXAMPLE. — What  size  conductor  should  be  used  for  the  open-wire 
transmission  shown  in  Fig.  130?  The  allowable  volts  loss  in  the  line  is 
4  per  cent.,  or  0.04  X  220  =  8.8  volts.  Receiver  voltage  =  220.  Load 
=  50  kw.  Power  factor  =  0.80.  Distance  between  wires  =  3  in. 
Frequency  is  25  cycles.  SOLUTION. — The  actual  current  in  each  wire 
must  be  known  to  insure  that  a  conductor  large  enough  to  carry  it  will  be 
selected. 


170  CENTRAL  STATIONS  [Axr.  199 

0.58  Xp       0.58  X  50,000       29,000 
Actual  current  =  „  xx  _  ,    =      220  x  0  8          -^-  :    165  amp. 


Now  find  one-half  of  the  total  load  and  proceed  with  this  load  as  for 
a  single-phase  transmission  which  will  be  called  the  imaginary 
transmission. 


M  total  1004  =  =  =  25,000  watts. 

The  current  in  the  imaginary  transmission  would  be: 

p  25,000      =  25,000  = 

~  E  X  p./.  ~  220  X  0.80  "     176 

142  amp.  in  the  imaginary  transmission. 


FIG.  130. — Example  in  determining  wire  size  for  a  three-phase  feeder. 
To  approximate  the  size  of  wire,  use  the  single-phase  formula  (54) : 

22  X  /  X  L       22  X  142  X  200       624,800     f 

Cir.mils  = =  —     —  =  =71,000  ar .  mils. 

V  8.8  8.8 

The  next  larger  standard  size  wire  is  No.  1 — 83,690  cir.  mils — which  will 
safely  carry,  when  exposed,  150  amp.  The  actual  current  is  165  amp. 
No.  1  is,  therefore,  not  satisfactory  from  a  current-carrying  standpoint. 
Hence,  it  will  be  necessary  to  use  the  next  larger  size  wire,  No.  0,  which 
will  safely  carry,  when  exposed,  185  amp.  Now  check  this  No.  0  wire 
for  volts  line  drop. 


The  average  distance  between  the  three  wires  = 

3  in.  +  3  in.  -f-  6  in.        12  in. 


4  in. 


Refer  to  Table  190.B  under  25  cycles  and  opposite  No.  0  wire  and  find: 
Resistance  volts  per  1,000  ft.  =  0.196  and  (under  4-in.  separation)  re- 
actance volts  per  1,000  ft.  =  0.066.  Then; 


SEC.  9] 
Resistance  drop 


ALTERNATING-CURRENT  CIRCUITS 
current  X  resist,  volts  X  dist. 


171 


oOO 


5.57 

Per  cent,  resistance  drop  =  ——  =  2.5  per  cent. 

current  X  react,  volts  X  dist. 
React,  drop  =  -^~ 


142  X  0.066  X  200 

-JSoo- 

Per  cenf.  reactance  drop  =  —  —  =  0.85  per  cent 


0657 


w  o.a  <xa  o.      s 

FIG.  131. — Illustrating  the  application  of  the  Mershon  diagram  for  computing 
a  three-phase,  three-wire  alternating-current  circuit. 

Laying  out  the  per  cent,  resistance  drop  and  the  per  cent,  reactance 
drop  on  the  Mershon  diagram  (Fig.  131):  At  the  upper  end  of  the  80 
per  cent,  power-factor  line  as  described  previously,  the  last  point  of  the 


172 


CENTRAL  STATIONS 


[ART.  200 


layout  comes  just  under  the  3  per  cent,  volts  loss  circle.  Therefore,  the 
true  volts  drop  in  the  line  will  be  somewhat  less  than  3  per  cent,  with 
No.  0  wire.  Therefore,  use  three  No.  0  wires  for  the  transmission  as 
shown  in  Fig.  130. 

200.  The  Determination  of  the  Wire  Size  of  Single-phase 
Branches  Fed  From  Three-phase  Mains  can  be  made  by  using 

...-javuts. 


Single  -Phase  Branches       _ 
(AtB+C  =  XAmp.  Totalh'' 

J 

A 

f    ,,ov.    i     n'4Amp-     j  ' 

1 

if  JK  ,iov.    n**»P.    j 

, 

^-'-"'•3  -Phase       ^^Amp.       \ 
Main                                    \ 

B 
0 

*  ,•  > 

\  8  c  a 

'n 

kr-J 

10 Amp." 


FIG.  132. — Current  in  three-phase  main  and  single-phase  branch. 

the  direct-current  formula.     The  branch  circuit  is  treated  as 
if  it  were  an  independent  single-phase  circuit. 

NOTE. — Fig.  132  shows  the  relation  of  the  total  current  in  a  three- 
phase  main  to  the  total  currents  taken  by  the  several  single-phase  branch 
circuits  feeding  from  it.  The  three-phase  circuit  current  is  equal  to  the 
total  of  the  single-phase  circuit  currents  multiplied  by  0.58.  Thus  for 
the  problem  of  Fig.  132:  30  amp.  X  0.58  =  17.4  amp. 


SECTION  10 

TRANSMISSION  AND  DISTRIBUTION  OF  ELECTRICAL' 
ENERGY 

201.  The  Reason  Why  Energy  Is  Transmitted  Electrically, 

particularly  where  large  amounts  are  to  be  transmitted  over 
long  distances,  is  that  the  electrical  method  is  the  most  eco- 
nomical, convenient,  simple  and  satisfactory  one  available  for 
the  applications  for  which  it  is  so  widely  used. 

NOTE. — Energy  may  be  transmitted  satisfactorily  and  in  some  cases 
most  economically  by  steam,  compressed  air,  line  shafts,  belt  and  rope 
drives  and  by  similar  methods.  But  it  is  obvious  that  any  of  the  methods 
just  mentioned  would  be  wholly  inadequate,  impracticable  and  uneco- 
nomical for  transmitting  large  amounts  of  energy  over  long  distances. 

202.  A  High  Voltage  Is,  From  a  Standpoint  of  Pure  Eco- 
nomics, Desirable  for  the  Transmission  or  Distribution  of 
Electrical  Energy. — However,  features  of  safety  and  utility 
often  render  desirable  or  necessary  the  use  of  relatively  low 
voltages.     Why  a  high  voltage  is  desirable  economically  will 
be  evident  from  a  consideration  of  the  articles  which  immedi- 
ately follow. 

203.  The  Power  Lost  in  an  Electrical  Circuit  Transmitting 
a  Given  Load  Varies  Inversely  as  the  Square  of  the  Impressed 
Voltage. — (Certain  factors  which  affect  only  very-high-voltage, 
alternating-current  lines  are  disregarded.)     If  the  voltage  im- 
pressed on  a  line  is  doubled  the  watts  line  loss — a  certain  given 
amount  of  power  being  transmitted — will  be  quartered.     If 
the  impressed  voltage  is  trebled  the  loss  will  be  one-ninth  of 
that  with  the  original  voltage.     Note  the  following  example: 

EXAMPLE. — Refer  to  Fig.  133.  In  both  /  and  II  the  same  line  is 
shown.  It  is  of  No.  10  B.  &  S.  gage  copper  wire,  which  has  a  resistance 
of  approximately  1  ohm  per  1,000  ft.,  so  the  entire  circuit  (10,000  ft.  of  wire) 
will  have  a  resistance  of  about  10  ohms.  The  load  at  the  end  of  the  line 
is,  in  each  case,  5  kw.  Since  the  apparatus  at  the  end  of  each  of  the  lines 
is  designed  for  operation  on  200  volts,  that  pressure,  approximately,  must 
be  maintained  at  the  receiving  end  of  each  line. 
173 


174 


CENTRAL  STATIONS 


[ART.  203 


In  system  7,  the  motor  is  connected  directly  to  the  line.  Hence,  the 
current  taken  by  the  motor  (which  will  be:  5,000  watts  -r-  200  volts  =  25 
amp.)  will  flow  in  the  line.  From  the  Ohm's  law  formulas,  the  voltage 
drop  in  the  line  for  system  A  will  be:  E  =  I  X  R  =  25  X  10  =  250 
volts.  And  the  power  loss  in  the  line  will  be:  P  =  7»  X  R  =  25  X  25  X 
10  =  6,250  watts. 

This  means  that  the  generator  voltage  would  have  to  be  equal  to: 
(volts  impressed  on  receiver)  +  (volts  loss  in  line)  =  200  +  250  =  450 
volts.  Note  also  that  a  certain  amount  of  power — 6,250  watts — is  lost, 
dissipated  as  heat,  in  the  line.  This  necessitates  that  the  generator  A 
develop  11,250  watts,  6,250  watts  more  than  is  delivered  to  the  motor. 
Obviously,  the  transmission  system  of  Fig.  7  is  not  an  economical  one 
because  more  power  is  lost  in  the  line  than  is  delivered  to  the  motor. 


•Law  Voltage  Generator 


^.-Transmission  Line  25  Amp. 


Motor Load  5  knt. 


-~-^4W  Volts 

,--/V-°  10  Wire 


DropmVb/tage  =  250  Volts 
Loss  in  Line*  6250  Watts 


U ^ WOOFER =10 Ohms- 

l«Transmission  at  '450  Volts 


•High  Voltage  Generator 


Drop  in  Voltage  =  25  Volts 
Loss  in  Line'  62.5  Watts 


-ZSAmp. 


U - -5000  Ft,  R= 10  Ohms "^ 

l-Transmtesion  at  2075  Volts 

FIG.  133. — Advantage  of  high- voltage  transmission. 

Now  consider  the  system  of  77  (Fig.  133).  The  line  and  the  motor 
are  the  same  as  in  7.  However,  a  transformer,  T,  is  inserted  at  the  end 
of  the  line.  This  transformer  is  so  designed  that  it  will  "step  down" 
from  a  pressure  of  2,000  volts  down  to  one  of  200  volts;  or  in  a  ratio  of 
10  to  1.  A  high-voltage  generator,  B,  is  connected  to  the  transmitting 
end  of  the  line.  Disregarding  the  small  losses  introduced  by  the  trans- 
former, the  current  in  the  line  will  be:  (5,000  watts)  -r  (2,000  volts)  =  2.5 
amp. 

From  the  Ohm's  law  formula,  the  voltage  drop  hi  the  line  for  system  B 
will  be:  E  =  I  X  R  =  2.5  X  10  =  25  volts.  The  power  loss  in  the  line 
will  now  be:  P  =  72  X  R  =  2.5  X  2.5  X  10  =  62.5  watts. 

The  generator  voltage  would  have  to  be:  2,000  +  50  =  2,050  volts. 
The  power  loss  in  the  line  is  now  only  62.5  watts.  Now  note  that  by 
increasing  the  voltage  10  times,  from  200  to  2,000  volts,  the  line  loss  was 
decreased  100  times.  This  example  shows  why  the  power  loss  hi  a  line 
varies  inversely  as  the  square  of  the  impressed  voltage.  It  was  decreased 
from  6,250  watts  to  62.5  watts.  The  use  of  alternating  current  in  B 


SEC.  10]     TRANSMISSION  OF  ELECTRICAL  ENERGY  175 

would  make  the  actual  results  slightly  different  from  those  obtained  in 
the  solutions  above,  but  the  difference  would  not  be  of  any  practical  con- 
sequence insofar  as  the  general  principle  described  is  concerned.  See  the 
author's  American  Electricians'  Handbook  for  an  example  tabulated  in 
detail  showing  the  effect  of  different  transmission  voltages  in  transmit- 
ting 30  kw.  over  a  line  of  3  ohms  resistance. 

204.  The  Weight  of  a  Conductor  Is,  for  a  Given  Power  Loss, 
Inversely  Proportional  to  the  Square  of  the  Impressed  Voltage. 

— The  truth  of  this  statement  may  be  readily  verified  by  solv- 
ing simple  examples  similar  to  these  above  given. 

205.  For  Short  Transmission  Distances  a  High  Voltage  is 
Seldom  Desirable  because,  although  the  cost  of  the  copper  in 
a  transmission  line  would  (with  a  given  watts  power  loss)  be 
less  than  if  a  low  voltage  were  used,  there  are  other  consider- 
ations which  more  than  offset  this  cost.     With  a  high  voltage, 
the  generator  is  frequently  more  expensive.     Furthermore, 
costly  transformers,  to  reduce  the  voltage  at  the  receiving  end 
of  the  line  must,  ordinarily,  be  used  for  incandescent  lighting 
and  also  for  motors  which  are  located  inside  of  buildings  where 
a  high  voltage  would  be  dangerous.     In  special  cases  relatively- 
high-voltage  motors  may  be  connected  direct  to  transmission 
lines  of  corresponding  voltages. 

206.  The  Efficiency  of  Transmission  of  an  Electrical  Cir- 
cuit is  similar  to  any  other  kind  of  efficiency  in  that  it  is  the 
ratio  of  output  to  input.     The  power  delivered  at  the  receiving 
or  far  end  of  any  electrical  circuit  is  always  less  than  the  power 
delivered  to  the  circuit,  by  an  amount  equal  to  the  losses  in 
the  line.     The  line  loss  is  almost  wholly  (and  for  practical 
purposes  may  be  considered  as  being  entirely)  the  P  X  R 
power  loss. 

NOTE. — Theoretically,  by  using  a  sufficiently  large  conductor,  the 
losses  may  be  reduced  to  practically  zero,  that  is  the  efficiency  may  be 
increased  to  almost  100  per  cent.  But  in  practice  nothing  is  gained  by 
using  an  excessively  large  conductor.  If  the  conductor  is  too  large,  the 
interest  on  the  money  invested  in  it  will  more  than  offset  the  cost  of  the 
energy  saved  by  using  the  large  conductor.  In  practice  transmission 
circuits  are  frequently  so  designed  that  they  are  about  90  per  cent. 
efficient;  that  is,  the  line  power  loss  is  about  10  per  cent.  It  is  impossible 
(Fig.  134)  to  have  a  circuit  which  is  exactly  100  per  cent,  efficient  regard- 


176 


CENTRAL  STATIONS 


[ART.  207 


less    of  how  large — of  how  little  resistance — its  conductors  are. 
designing  circuits*  every  case  should  be  treated  on  its  merits. 


In 


207.  To  Compute  the  Efficiency  of  a  Transmission  Line  or 

circuit,  a  formula  based  on  the  preceding  statements  may  be 
used.  In  general:  efficiency  =  output  -f-  input.  Stating  the 
same  thing  in  another  way:  efficiency  =  (output)  -r-  (output  + 
Now,  restating  this  to  apply  to  a  transmission  line: 


110 

*%: 

^ 

—  W* 

| 

c 

J  90 
£80 

'-I* 

! 

>fp9/ 

-ffr86 
•h-\i5% 

K97.S% 

\ 

% 

9?. 

7: 

| 

\ 

L--63.3% 

"i  - 

r 

ffic 

H.    Tr 
?JKC 

enc 

jn;rr 
e  at 

j  of  Trar 
it  teef  ovei 
Differen 

emission  Line  — 
a  Line  of  Z  Ohms 
Impressed  Voltages 

.-41 

3% 

30* 

Res, 

Efficiency  of 

| 

1 

A 

3             1000           2,000           3000         4000           5000          ($00           1,000          8.000          9,000        10.00 
Voltage  Impressed  by  Generator  on  Line 

FIG.  134. — Graph  showing  increase  of  transmission  efficiency  with  the 
transmission  voltage.  (The  values  plotted  above  relate  to  the  specific 
problem  designated.  Note  that  in  this  particular  example  the  efficiency  of 
transmission  increases  very  rapidly  with  increase  of  impressed  voltage  until 
the  pressure  is  (at  B)  above  1200  or  1500  volts.  Above  this  pressure  efficiency 
increases  much  more  slowly.  Even  at  C,  for  a  pressure  of  10,000  volts,  the 
efficiency  is  only  99.9  per  cent.) 


(66)  Effi.  of  transmission 
or  modified : 

(67)  Efficiency  of  transmission 


power  delivered  by  line  , 

— : — ,  ,     T. —  (per  cent.) 
power  received  by  line 


power  del.  by  line 


(power  del.  by  line)  +  (power  losses  in  line)' 

EXAMPLE. — What  is  the  efficiency  of  the  circuit  of  Fig.  135  under  the 
conditions  there  noted?  The  impressed  e.m.f .  is  112  volte.  The  current 
is  800  amp.  The  resistance  of  each  line  conductor  is  0.006  ohm.  SOLU- 

•  See  article  headed  "  The  Question  of  Energy  Loss  in  a  Circuit "  in  the  author's 
AMERICAN  ELECTBICIANS'  HANDBOOK. 


SEC.  10]     TRANSMISSION  OF  ELECTRICAL  ENERGY  177 

TJON.— Power  received  by  the  line  =  /  X  E  =  800  X  112  =  89,600  watts. 
Power  lost  in  line  =  72  X  R  =  800  X  800  X  0.012  =  7,680  watts. 
Hence,  power  delivered  by  line  =  89,600  -  7,680  =  81,920  watts.  Now 
substitute  in  formula  (66) : 

power  del.    by  line      81,920 
W'  *  trans-  -  e  =         00  =  °'913  =  °L3   ^   ^ 


••Generating  Station 
•HZ  Volts 


•power  rec.  by  line      89,600 

Center  of  Distribution., 


,  •<—  I  =800  Amp. 

"-Resistance  of  each 
Conductor  is  0.006  Ohms 


FIG.  135. — Example  in  computing  efficiency  of  transmission. 

208.  Direct  Current  for  Transmission  and  Distribution  is 

with  multiple  circuits,  ordinarily,  suitable  only  where  the  dis- 
tances involved  are  not  great.  Direct-current  voltages  can 
be  "stepped  up"  or  down  only  by  using  motor  generators 
which  are  expensive  in  first  and  operating  costs.  Hence,  the 
economics  of  the  situation  dictate  that,  as  a  rule,  the  receivers 
— lamps  and  motors — on  direct-current  multiple  circuits  must 
operate  at  the  voltage  which  is  impressed  on  the  circuit  by 
the  direct-current  generator. 

Because  of  the  fact  that  multiple  incandescent  lamps  de- 
signed for  operation  on,  approximately,  110  volts  are  more 
economical  in  first  and  operating  costs  than  lamps  for  higher 
voltages  this  pressure,  110  volts,  is  practically  standard  for 
incandescent-lighting  circuits.  Lamps  for  voltages  lower  than 
110  would  of  themselves  be  satisfactory,  but  the  cost  of  the 
large  conductors  which  would  be  necessary  to  serve  low-voltage 
lamps  with  energy  would  be  prohibitive.  The  result  is  that 
the  pressure  on  any  circuit  used  for  incandescent  lighting  is, 
in  effect,  limited  to  approximately  110  volts.  Obviously  this 
pressure  is  not  sufficiently  high  for  economically  transmitting 
energy  over  considerable  distances. 

209.  Relatively-high-voltage  Direct-current  Transmission 
and  Distribution  is  satisfactory  and  economical  where  the  load 

12 


178 


CENTRAL  STATIONS 


[ART.  210 


is  mainly  power  (motors)  rather  than  multiple  incandescent 
lighting.  Direct-current  pressures  as  high  as  1,500  and  3,000 
volts  have  been  applied  successfully  for  electric  railway  work. 
In  industrial  plants  550  volts,  direct  current,  has  been  used 
to  some  extent  for  motors  and  cranes;  higher  voltages  are  un- 
desirable for  general  distribution  within  buildings  or  plants, 
because  of  the  danger  to  human  life  that  they  involve. 

NOTE. — Voltages  lower  than  550  may  be  fatal.  Where  the  contact 
with  the  body  is  good  or  where  the  person  has  a  weak  heart,  a  voltage  as 
low  as  110  may  kill 


-Generating  Station 


-X 

Switch 


Copper  Conduc  tors  •'•'' 


E-  Receive  </oits--.,~_ 
Sub  Statia 

Distribution  Panel-  •> 
Load  =  50  Kw. 


Property 


1=  Current  in  ^  he  Li 


V-  Voltage  Drop  in  Li 


R  =  Resistance  of  Line 


Cir.  Mils  =  Area  of  Wire 


E  =  I10  Volts 


2.5%  of  110  =  2.75  Volts 


E=  220  Volts 


2.5%  of  220  =  5.5  Volts 


Fio.  136. — Comparison  of  110-  and  220-volt  transmission,  adapted  from 
Gray.  (This  table  shows  the  solution  of  a  problem  where  50  kw.  must  be 
transmitted  300  feet  with  a  2.5  per  cent,  drop  and  where  it  is  required  that  the 
conductor  size  be  known  for  a  receiver  pressure  of  110  volts  and  also  220  volts. 
The  resistivity  of  copper  is  taken  as  11  ohms  per  circular  mil  foot.  Note 
that  since,  with  the  same  per  cent,  line  loss  and  the  same  load,  the  conductor 
area  varies  inversely  as  the  square  of  the  transmission  voltage,  the  area  with 
E  =  110-volts  is  four  times  that  with  E  =  220  volts.  In  other  words,  by 
doubling  the  pressure  the  conductor  area  has  been  quartered.) 

210.  Three -wire  Distribution  is  now  used  in  practically  all 
installations  of  any  consequence  where  multiple  incandescent 
lamps  are  to  be  served.  With  the  three- wire  system,  110-volt 
lamps  may  be  used  on  the  side  circuits  while  the  energy  is,  in 
effect,  transmitted  by  the  outside  wires  at  220  volts.  Thereby 
the  economics  of  220-volt  transmission  (see  Fig.  136)  are 
utilized.  The  neutral  wire  may  be  small  where  the  load  on 
the  three-wire  circuit  is  well  balanced.  The  consequence  is 


SEC.  10]     TRANSMISSION  OF  ELECTRICAL  ENERGY 


179 


that  the  weight  of  copper  conductor  required  for  a  three-wire 
system  will  be  only  a  quarter  to  three-eighths  of  that  necessary 
for  an  equivalent  two-wire  system. 

211.  Standard  Direct-current  Voltages  and  Their  Applica- 
tions. 


Voltages 

Applications 

Generators    and 
energy-delivering 
apparatus 

Motors   and 
energy-utilization 
apparatus 

*125 

110 

Used  for  multiple-circuit,  incandescent 
lighting.    Usually  obtained  from   a 
110-220-volt  three-wire  system. 

*125-*250 
575-*600 

f     110-  220  ) 
{   *115-*230  \ 
(       *550       J 

Direct-current  motors. 

*600 

Urban  and  interurban  electric  railways. 

1,200 
1,500 

..   1 

Interurban  railways. 

.   } 

2,400 
3,000 

.-   \ 

Trunk  line  railways. 

.   } 

*  Electric  Power  Club  standard  voltage  ratings. 

NOTE. — VOLTAGES,  SYSTEMS  AND  FREQUENCIES  IN  USE  IN  THE  UNITED 
STATES. — According  to  a  recently  published  electrical  directory  of  the 
United  States,  *  there  are  no  less  than  4,700  central  stations  in  towns 
of  less  than  50,000  inhabitants.  An  analysis  of  the  data  relative  to  576 
systems  in  six  representative  States  indicates  that  15  per  cent,  use  direct 
current  at  voltages  of  125,  250  and  550  volts  two-wire,  and  125  to  250 
volts  three-wire.  How  well  some  of  these  direct-current  systems  are 
applied  is  not  apparent,  but  it  is  evident  that  the  voltage  of  direct- 
current  systems  is  quite  well  standardized.  Of  the  total  number  of 
systems  analyzed,  85  per  cent,  employed  alternating  current  in  22  differ- 
ent combinations  of  number  of  phases,  wires,  cycles  and  volts,  prac- 
tically none  of  which  are  convertible  into  another  combination  without 
great  difficulty  and  expense. 

•  A.  J.  Goedjen  in  a  paper  read  before  a  joint  meeting  of  the  Electrical  Section  of 
the  Western  Society  of  Engineers  and  the  Chicago  Section  of  the  American  Institute 
of  Electrical  Engineers. 


180  CENTRAL  STATIONS  [ART.  212 

In  the  488  alternating-current  systems  analyzed,  24  per  cent,  were 
single-phase,  13  per  cent,  three-wire  two-phase,  1  per  cent,  four-wire 
two-phase,  39  per  cent,  three-wire  three-phase,  and  23  per  cent,  four-wire 
three-phase.  Of  these  488  systems,  classified  according  to  the  voltages, 
2  per  cent,  operate  at  115  volts,  17  per  cent,  at  1,100  volts,  58  per  cent, 
at  2,200  volts  and  24  per  cent,  at  4,000  volts.  The  frequencies  at  which 
the  488  systems  operate  show  even  a  greater  diversity,  2  per  cent,  being 
25-cycle,  0.6  per  cent.  40-cycle,  90  per  cent.  60-cycle,  0.2  per  cent.  116- 
cycle,  1  per  cent.  125-cycle,  and  6.2  per  cent.  133-cycle.  The  116-,  125- 
and  133-cycle  systems  are  small  plants. 

212.  Alternating-current    Transmission    and    Distribution 
Systems  (Figs.  1  and  137)  are  not,  in  general,  restricted  by 
distance  conditions.     It  is  economically  feasible  to  transmit 
almost  any  value  of  power  over  any  reasonable  distance  with 
alternating  current.     Hence,  where  it  is  necessary  that  power 
be  transmitted  further  than  a  mile  or  so  or  be  distributed  over 
a  wide  area  for  general  light  and  power  service  the  alternating- 
current  system  is  always*  adopted — because  it  is  far  more 
economical   than   a   direct-current   system.     The   cost   of   a 
multiple-circuit,    direct-current    long    distance    transmission 
and  distribution  system  would  be  prohibitative. 

213.  The  Reason  Why  Alternating  Current  Is  in  General, 
Preferable  for  Transmission  and  Distribution  is  that  the 
voltages  may  be  transformed    ("stepped  up"   or  "stepped 
down")   with  stationary  transformers.     These  transformers 
are  relatively  low  in  cost,  and  are  very  efficient  in  operation. 
They  have  no  moving  parts  hence  require  no  attendance 
and  may  be  installed  out  of  doors,  in  manholes  or  on  poles. 
Hence,  energy  may  be  generated  at  the  most  convenient  vol- 
tage, increased,  with  a  step-up  transformer,  to  a  pressure  high 
enough  for  economical  transmission  or  distribution  and  then 
lowered  at  the  distant  end  of  the  line,  with  a  "step-down" 
transformer  to  a  voltage  or  voltages  suitable  for  effective 
utilization. 

EXAMPLE. — Figs.  1  and  138  show  typical  three-phase  systems.  In 
the  system  of  Fig.  138  energy  is  generated  by  the  alternator  G  at  2,200 
volts.  The  voltage  is  "stepped  up"  by  transformers  Tu,  located  in  the 

*  See  the  author's  AMERICAN  ELECTRICIANS'  HANDBOOK  for  table  showing  Copper 
Economics  of  different  distribution  systems. 


SEC.  101     TRANSMISSION  OF  ELECTRICAL  ENERGY 


181 


182 


CENTRAL  STATIONS 


[ART.  214 


generating  station,  to  50,000  volts  which  is  the  transmission  pressure. 
The  energy  is  then  transmitted  at  50,000  volts  from  the  generating  station 
(Li)  over  the  transmission  line  for  a  distance  of  possibly  25,  50  or  100 
miles,  to  the  receiving  station  (Z/z) .  At  the  receiving  station  the  pressure 
is  stepped  down  with  stationary  transformers  to  13,200  volts  at  which 
pressure  it  is  distributed  to  substations,  which  may  be  of  any  one  of  the 
four  types  enumerated  below  or  a  combination  thereof. 


Power  House  ««»  ™ts  to  Sub-Station— > 

Terminal  Station 

FIG.  138. — The    elements   of   a   high-voltage   alternating-current   electrical 
energy  transmission  system. 


214.  Three-phase  Transmission  Is  Used  hi  Preference 
to  Single-phase  or  Two-phase  because  it  is  more  economical 
of  copper.*  If  a  single-phase,  two-wire  transmission  operat- 
ing at  a  certain  voltage  requires  a  certain  amount  (or  100  per 
cent.)  of  copper,  an  equivalent  two-phase,  four-wire  trans- 
mission will  also  require  100  per  cent.  An  equivalent  three- 
wire,  three-phase  transmission  will  require  only  75  per  cent, 
of  the  copper.  A  four-wire,  three-phase  transmission  with 
the  neutral  the  same  size  as  the  outers  will  require  only  33.3 
per  cent. 


*  See  the  author's  AMERICAN  ELECTRICIANS' 
Economics  of  different  distribution  systems. 


IAXDBOOK  for  table  showing  Copper 


SEC.  10]     TRANSMISSION  OF  ELECTRICAL  ENERGY  183 

215.     Standard  Alternating-current  Voltages  and  Their  Applications. 


Voltage 

Application 

Generators  and 
ene  rgy-deli  veri  ng 
apparatus 

Motors    and 
energy-utilization 
apparatus 

120 
*240 

*110 

Single-phase,  used  for  small  motors  and 
lighting,  usually  obtained  from  a  120- 
240-volt  three-wire  system. 

*240 
*480 
*600 

*110-*220 
*440-*550 

Usually  three-phase,  used  for  distribu- 
tion for  power  for  polyphase  motors 
up  to  possibly  50  to  60  h.p.  output. 

*1,200-  *2,400 
1,150     2,300 

*440-*550 
*1,100-*2,200 

Usually  three-phase,  used  for  poly- 
phase motors  of  capacities  greater 
than  about  50  to  60  h.p. 

2,300-4,000 
*2,  400-4,  150 



For  three-phase  four-wire  distribution 
in  cities,  4,000  volts  between  outer 
wires  and  2,300  volts  between  outer 
wire  and  neutral. 

fl3,200 
See  footnote  A. 

Highest  pressure  for  which  generator  or 
motors  can,  ordinarily,  be  effectively 
designed.  Often  better  to  generate 
at  2,200  volts  and  then  stop  up  with 
transformers  to  transmission  line 
voltage. 

t  6,600 

tn,ooo 

113,200 

tie,  500 

f22,000 

The   voltages 
higher   than 
13,200  are  used 
for  transmis- 
sion only  and 
not  for  genera- 
tion 

For  power  transmission  over  relatively 
short  distances  and  where  cable  must 
be  used. 

f22,000 
f33,000 
144,000 
f66,000 
t88,000 

n  10,  ooo 

fl50,000 

For  long-distance  power  transmission 
over  aerial  lines.  See  "Thousand 
Volts  Per  Mile"  article  below. 

A.  Generators  are  sometimes  built  for  4,000,  6,600  and  11,000  volts. 
*  Electric  Power  Club  standard  voltage  ratings. 
t  These  voltages  standardized  by  the  National  Electric  Light  Associa- 
tion and  the  Electric  Power  Club. 


184  CENTRAL  STATIONS  [ART.  216 

216.  About  a  Thousand  Volts  Per  Mile  Length  of  Trans- 
mission* is  a  thumb  rule  which  serves  as  an  index.     This  is 
based  on  the  fact  that  with  copper  conductors  a  pressure  of 
1,000  volts  per  mile,  and  a  current  density  of  1  amp.  per  1,000 
cir.  mils,  the  energy  loss  will  be  about  10  per  cent.     That  is, 
a  line  designed  on  this  basis  will  carry  its  current  without 
excessive  heating  at  about  a  10  per  cent.  loss.     For  relatively 
short  lines  transmitting  considerable  power  the  rule  provides 
a  conductor  too  small  for  most  economical  operation,  that  is 
for  minimum  annual  costs.     Hence,  for  the  short  distance 
transmission  of  much  power,  a  pressure  greater  than  1,000 
volts  per  mile  may  be  the  more  economical.     Where  a  trans- 
mission system  serves  an  extensive  distance  and  the  load  is 
small  a  voltage  smaller  than  "1,000  per  mile"  may  sometimes 
be  adopted  with  economy. 

217.  The  Standard  Frequency f  in  the  United  States  may 
now  be  said  to  be  60  cycles.     It  appears  that  it  is  desirable, 
in  practically  every  case,  to  adopt  this  rather  than  any  other 
frequency.     The  economies  and  advantages  that  were  expected 
to  result  from  the  use  of  25  cycles  for  electrical  energy  trans- 
mission have  not,   in  general,   materialized.     A  number  of 
plants  which  generate  at  25  cycles  have  been  constructed  which 
necessitates    the    installation   of  25-  to   60-cycle  frequency 
changers  where  60  cycles  is  the  utilization  frequency. 

NOTE. — Synchronous  converters  as  formerly  designed  would  not 
operate  satisfactorily  at  frequencies  much  above  40  cycles.  This  was 
one  of  the  reasons  for  the  original  installation  of  a  number  of  25-cycle 
generating  stations.  Motors  for  25  cycles  are,  generally,  except  very 
slow-speed  machines,  more  expensive  than  those  for  60  cycles.  Arc 
lights  can  not  be  used  on  25  cycles.  With  incandescent  lamps  there  is  a 
noticeable  flickering  on  25  cycles  which  produces  eye  fatigue.  This 
renders  its  application  undesirable  for  interior  illumination  but  it  can 
be  used  for  incandescent  street  lighting. 

218.  Sub-stations  May,  in  General,  Be  Divided  into  Four 
General  Classes:     (1)  Transformer  sub-stations,  (2)  rotary- 

*  H.  B.  Gear. 

t  See  the  author's  AMERICAN  ELECTRICIANS'  HANDBOOK,  his  PRACTICAL  ELECTRICITY 
and  his  ELECTRICAL  MACHINERY  for  further  information  on  this  subject. 


SEC.  10]     TRANSMISSION  OF  ELECTRICAL  ENERGY 


185 


converter  sub-stations;  (3)  motor-generator  sub-stations,  and 
(4)  frequency-changer  sub-stations. 

218A.  The  Function  of  a  Sub-station  Equipment  is  to  so 
modify  the  characteristics  of  the  energy  received  by  it  that 
the  energy  will  then  be  suitable  for  utilization  by  that  load 
which  the  sub-station  serves.  That  is,  the  voltage  may  be 
lowered  in  the  sub-station  and  a  conversion  made  from  alter- 
nating to  direct  current,  if  necessary,  as  will  be  described. 
So  that  each  sub-station  may  most  economically  serve  its 
load  it  should  be  located  at  or  near  the  electrical  center  of  the 
district  served. 


FIG.  139. — Typical  transformer  sub-station. 

219.  A  Transformer  Substation  (Fig.  139)  is  one  in  which 
the  alternating-current  voltage  is  lowered,  with  step-down 
transformers,  from  the  transmission  voltage  to  one  suitable 
for  distribution  to  the  consumers  or  to  the  power  load.  Usu- 
ally the  distribution  primary  feeders  operate  at  2,200  volts. 
Hence,  the  low-tension  side  of  the  step-down  transformer 


186 


CENTRAL  STATIONS 


[ART.  220 


develops  this  voltage.  A  potential  or  feeder  regulator,* 
which  may  be  automatic  or  non-automatic,  is  usually  inserted 
in  each  feeder  to  maintain  the  voltage  at  the  distant  end  of 
the  feeder  practically  constant.  In  a  transformer  sub-station 
the  pressure  is  transformed  from  one  voltage  to  another  but 
the  energy  is  not  converted  from  alternating  to  direct  current 
or  the  reverse. 

220.  In  a  Synchronous  or  Rotary  Converter  Sub-station 
(Figs.  140  and  141)  conversion  from  alternating  to  direct  cur- 


FIG.  140. — Sectional  elevation  showing  typical  arrangement  of  a  synchronous 
converter  sub-station  for  electric  railway  service. 

rent  is  effected.  Usually  the  voltage  must  be  decreased  with 
a  transformer  on  the  alternating-current  side  of  the  synchron- 
ous converter  because  there  is  a  certain  fixed  ratio  between  the 
alternating  voltage  impressed  on  a  synchronous  converter  and 
the  direct  voltage  delivered  by  it.  With  a  single-phase  con- 
verter, the  alternating  is  71  per  cent,  of  the  direct  voltage. 
With  a  three-phase  machine  the  alternating  is  61  per  cent,  of 
the  direct  voltage.  Hence,  to  change  the  direct  voltage  de- 
livered by  the  converter,  the  alternating  must  be  varied  ac- 

*  See  author's  PSACTICAI,  ELECTRICITY. 


SEC.  10]     TRANSMISSION  OF  ELECTRICAL  ENERGY 


187 


cordingly.  By  changing  the  field  excitation,  a  converter  may 
be  made  to  correct  or  compensate  for  low  power  factor.  The 
direct  e.m.f.  impressed  on  the  line  by  a  synchronous  converter 
may  be  varied  by  using  a  booster — a  small  generator — either 


I- Plan  View 
FIG.  141. — Plan    view    of   the   railway,  synchronous-converter   sub-station. 

in  the  alternating-  or  direct-current  side  of  the  machine,  or  by 
varying  the  alternating  impressed  voltage  with  a  potential 
regulator  or  a  transformer  having  taps. 

NOTE. — Synchronous  converters  are  somewhat  more  efficient  than 
motor-generators.  They  find  their  widest  application  in  direct-current 
street  railway  service. 

221.  A  Motor-generator  Sub-station  is  shown  in  Fig.  142,7 
and  //.  Such  an  outfit  may  be  used  particularly  in  industrial 
plants,  where  direct-current  energy  is  required  for  utilization. 


188 


CENTRAL  STATIONS 


[ART.  221 


The  high-voltage  alternating  three-phase  line  enters  the  sta- 
tion over  three  wires,  X,  Y  and  Z,  and  passes  through  the  choke 
coil,  C,  to  the  high  tension  bus-bars  1,  2  and  3.  The  line  vol- 


,-D.C.  Switchboard 


Choke 
.-Coils 


^Synchronous 
UotorPanels 


D.C.  •'  'Self-Starting 

Generator          Synchronous 
Motor 


FIG.  142. — Typical    arrangement   of   a   synchronous   motor-generator   sub- 
station. 

tage  is  stepped  down  to  2,200  volts  by  the  three  single-phase 
transformers,  2\,  Tz  and  2%  which  are  connected  in  delta. 
The  2,200-volt  alternating  power  drives  the  synchronous 
motor,  M ,  which  is  mounted  on  the  same  shaft  with  and  drives 


SEC.  10]      TRANSMISSION  OF  ELECTRICAL  ENERGY 


189 


the  direct-current  generator  G.  The  synchronous  motor 
usually  has  a  squirrel-cage  winding  on  its  rotor  and  is  thereby 
started  as  an  induction  motor.*  The  direct  e.m.f.  impressed 
on  the  line  by  G  may  be  made  any  reasonable  one  by  providing 
a  generator  of  suitable  characteristics,  and  it  may  be  controlled 
manually  by  a  field  rheostat  or  automatically  with  an  auto- 
matic voltage  regulator.  Motor-generators  are  sometimes 
preferred  to  synchronous  converters  because  the  motor- 
generators  are,  possibly,  somewhat  more  readily  operated. 
The  synchronous-converter  outfits  are  the  more  efficient. 

222.  A  Frequency-changer  Sub  -station  is  one  in   which 
alternating-current  power  at  one  frequency  is  changed  to  al- 
ternating-current   power   at   different   frequency.     The    fre- 
quency changer  sub-station  is  somewhat  similar  to  the  syn- 
chronous-motor   sub-station    of   Fig.    142   except   that    the 
direct-current  generator  and  its  switching  and  control  equip- 
ment  is   replaced   by  an  alternating-current  generator  and 
outfit.*    Frequency-changer    stations  in   the   United  States 
ordinarily  change  from  60  to  25  cycles  or  the  reverse. 

223.  Distribution  Circuits  may  for  convenience  be  divided 
into  the  following  two  general  classes:  (1)  Series  circuits  (Fig. 
143);  and  (2)  parallel  circuits  (Figs.  144  to  148).     Parallel 
circuits  may  be  subdivided  into:  (a)  loop  circuits  (Fig.  144), 
(6)  tree  circuits  (Figs.  145  and  146),  (c)  feeder-and-main  cir- 
cuits (Fig.  147),  and  (d)  ring  circuits  (Fig.  148). 


<— 6.6  Amp. 

•  Total  Length  of  Circuit*  5  Miles 
FIG.  143. — Arc-lighting  circuit. 


224.  Series  Distributing  Circuits  (Fig.  143)  are  seldom,  if 
ever,  used  in  this  country  except  for  constant-current  J  series 
arc  or  series  incandescent  lighting.  The  constant  current  in 


*  See  the  author's  ELECTRICAL  MACHINERY  for  details. 
%  See  the  author's  PRACTICAL  ELECTRICITY. 


190  CENTRAL  STATIONS  [ART.  225 

commercial  series  circuits  is  so  small  (usually  under  8  amp.) 
that  a  small  wire  will  carry  it  without  excessive  loss.  Hence, 
conductors  for  series  circuits  are  usually  selected  with  reference 
only  to  their  mechanical  strength.  It  is  usually  considered 
that  No.  6  wire  is  as  small  as  should  be  erected  on  a  pole  line, 
hence  a  majority  of  the  out-of-door  series  lighting  circuits  in 
this  country  are  of  No.  6  B.  &  S.  triple-braid  weather-proof 
copper  wire.  Some  companies  will  not  use  any  wire  smaller 
than  No.  4  B.  &  S.  on  a  pole  line. 

225.  The  Line  Loss  in  Commercial  Series  Circuits,  using 
the  standard  No.  6  wire  is  relatively  small  as  is  indicated  by 
the  following  example. 

EXAMPLE*  (see  Fig.  143). — The  circuit  operates  at  6.6  amp.,  is  5  miles 
long  and  serves  80  lamps,  each  of  which  requires  50  volts  at  its  terminals. 
The  line  is  of  No.  6  wire  which  has  a  resistance  of  2.1  ohms  per  mile  or 
10.5  ohms  for  the  whole  line.  This  involves  a  drop  of  (V  =  IX  R) 
10.5  ohms  X  6.6  amp.  =  69.3  volts.  The  loss  of  energy  in  the  line  wire 
is:  (P  =  72  X  R)  =  6.6  X  6.6  X  10.5  =  468  watts.  The  power  taken 
by  one  lamp  (7  =  E  X  /)  is:  50  X  6.6  =  330  watts  and  for  the  80  lamps 
it  would  be  80  X  330  =  26,400  watts.  The  loss  in  the  line,  468  watts,  is: 
(468  -s-  26,400)  but  1.8  per  cent,  of  the  power  taken  by  the  arc  lamps. 

If  No.  3  wire  were  substituted  for  No.  6,  one-half  the  energy  loss  in  the 
line  wire  would  be  saved,  the  cross-section  and  weight  being  twice  as 
great,  and  the  cost  of  the  insulated  conductor  would  be  nearly  doubled. 
With  No.  6  wire  the  total  weight  of  copper  would  be  2,098  Ib.  and  the 
cost  of  the  wire  (insulated)  would  be  about  $600.  It  is  doubtful  if  it 
would  be  wise  to  invest  an  additional  $500  in  order  to  use  No.  3  wire  and 
save  one-half  the  energy  or  238  watts,  unless  the  cost  of  energy  is  quite 
high. 


.-.Loop-Circuit  Conductors 


Incandescent  Lamps' 


FIG.  144.  —  Diagrammatically  illustrating  a  loop  circuit. 

226.  Parallel  Distributing  Circuits  (Figs.  144  to  148),  some- 
times called  multiple  circuits,  are  widely  used  for  the  distribu- 
tion of  electrical  energy  for  lighting,  power  and  heating.  Com- 

*  Crocker's  ELECTRIC  LIQHTINO- 


Sic.  10]     TRANSMISSION  OF  ELECTRICAL  ENERGY  191 


:,v^;.^;:> 

Ashley 


B 


FIQ.  145. — Showing  a  tree  circuit  as  applied  to  out-of-door  distribution. 
(This  type  of  a  circuit  is  called  a  "tree  circuit"  because  of  its  resemblance 
to  the  trunk  and  branches  of  a  tree.) 


Switch 
,-anctCutOut 


-Wallof 
Building 

Sub-Main^ 


Incandescent1 
Lamps* 


FIG.  146. — An  interior  distribution  "tree"  circuit. 


Station  or  Sour 
Of  [IK  tricot 
.-Energy 

ze 

\                              J_ 

I 

1 

Jj    _t« 

feeders                                     _ 

E 

l£Ffi 

KA/a/ns-> 

r. 

] 

1 

D 

A 

I 

| 

~ 

- 



I 

distribution  Cen 

rer-' 

L 

I 

FIQ.  147. — Example  of  a  feeder  and  main  distribution. 


192 


CENTRAL  STATIONS 


[ART.  227 


mercial  parallel  circuits  are  so  designed  that  the  voltage  be- 
tween the  two  sides  will  be  approximately  constant  under  all 
conditions  of  load.  That  is,  sufficient  copper  is  used  to  pre- 
vent the  voltage  drop  in  them  from  exceeding  a  certain  small 
percentage  of  the  receiver  voltage  which  should  in  each  case 
be  determined  by  the  character  of  the  connected  apparatus. 


FIG.  148. — Diagrammatically  illustrating  a  ring  circuit. 

Or  a  voltage  regulator  of  some  sort  is  used  to  maintain  the 
voltage  at  the  load  ends  of  the  feeders  approximately  constant. 
In  dealing  with  parallel  circuits,  it  is  frequently  assumed  that 
the  voltage,  impressed  on  the  circuit  by  the  generator  or  other 
source  of  energy,  is  constant. 


Note:-  Each  Single  Line 
Represents  a  Circuit 


FIG.  149. — Feeders,  mains  and  services. 

227.  An  Important  Advantage  of  the  Feeder-and-main  Sys- 
tem (Fig.  149)  is  the  opportunity  it  offers  for  close  voltage 
regulation  at  the  receivers.  Receiving  apparatus  is  not  con- 


SEC.  10]     TRANSMISSION  OF  ELECTRICAL  ENERGY  193 

nected  to  the  feeders  so  the  voltage  regulation  on  them  is  un- 
important. The  regulation  on  the  mains  and  services  is  im- 
portant but  they  are  made  of  wire  sufficiently  large  that  there 
will  not  be  much  voltage  drop,  even  at  full-load,  between  the 
distribution  center,  where  the  mains  connect  to  the  feeder, 
and  the  most  distant  point  on  the  main.  The  voltage  should 
be  maintained  practically  constant  at  the  distribution  center. 

228.  Small  Pressure  Wires  Are  Sometimes  Carried  From 
the  Distribution  Center  Back  to  the  Generating  or  Sub-station 
so  it  is  possible  at  any  time  to  know  exactly  the  voltage  at 
the  distributing  center.     Then  the  voltage  at  the  center  is 
maintained  constant  by  varying  the  voltage  impressed  on  the 
feeder  at  the  station.     Frequently  the  voltage  at  the  distribu- 
tion center  is  thus  maintained  constant  with  a  potential  regu- 
lator.*    The  varying  or  adjustment  of  voltage  may  be  either 
manual  or  automatic. 

229.  In  Laying  Out  an  Out-of-door  Feeder-and-main  Cir- 
cuit the  territory  to  be  served  is  subdivided  into  a  number  of 
districts  by  collecting  customers  in  proximity  to  each  other 
into  groups  located  as  near  as  may  be  at  equal  distances  from 
a  number  of  distributing  centers.     From  these  centers,  the 
feeders  are  carried  to  the  station  while  from  the  centers  extend 
the  mains,  each  of  which  serves  its  own  groups  of  customers. 
Thus  the  entire  territory  is  split  up  into  a  number  of  subdivi- 
sions, each  in  the  most  direct  electrical  connection  with  the 
central  station.     Each  distribution  center  is  fed  by  a  separate 
and  independent  set  of  feeders. 

230.  In  Interior  Feeder-and-main  Systems  it  is  seldom 
feasible  to  regulate  the  voltage  on  the  supply  ends  of  the  feeders 
so  as  to  keep  it  constant  at  the  center.     But  most  of  the  vol- 
tage drop  in  the  interior  system  can  be  confined  to  the  feeders 
so  that  the  voltage  on  the  group  of  mains  and  branches  served 
by  a  given  feeder  will  be  nearly  the  same  and  all  of  the  lamps 
connected  to  them  will  burn  at  about  the  same  brightness. 
Furthermore,  the  system  is  so  sectionalized  that  the  effects 
of  trouble  can  be  confined  to  small  areas  and  that  the  trouble 
can  be  readily  located. 

*  See  the  author's  PRACTICAL  ELECTBICITT. 
13 


194  CENTRAL  STATIONS  [ART.  231 

231.  A  Ring  Circuit,  Fig.  148,  is  one  wherein  the  mains  form 
a  closed  ring.  This  is  a  special  case  of  a  feeder-and-main 
circuit.  In  out-of-door  distributions  ring  mains  are  sometimes 
carried  around  a  city  block  or  around  a  certain  district  and 
branch  mains  or  services  are  fed  by  the  ring  main.  One  feeder 
may  serve  a  ring  main  or  several  may  connect  to  it,  each  at  a 
different  point.  In  interior  electrical-energy  distributions, 
ring  mains  are  seldom  used  except  in  industrial  plants.  It  is 
sometimes  expedient  to  carry  a  ring  main  around  the  interior 
of  a  shop  building  and  connect  motor  taps  and  lighting 
branches  to  it  at  the  most  convenient  points  (Fig.  148).  This 
provides  a  very  flexible  arrangement  because  there  is  then  no 
location  in  the  building  very  far  away  from  the  main,  hence 
new  motors  or  lights  can  be  installed  readily  and  economically. 


SECTION  11 
LIGHTNING  PROTECTION  APPARATUS 

232.  The  Term  Lightning  Protector  Is  Used  in  Preference 
to  Lightning  Arrester  in  this  discussion  because  it  appears  that 
the  latter  designation  more  accurately  describes  the  service 
which  the  apparatus  in  question  renders.     The  word  "arrest," 
according  to  the  dictionary,  means  to  "stop  action  of."     The 
so-called  lightning  arresters  do  not  in  every  case  stop  the  action 
and  effects  of  lightning  nor  does  any  manufacturer  make  the 
claim    that    they   are   infallible.     They  do,  however,  afford 
protection  or  insurance  against  lightning  damage  to  electrical 
apparatus.     The  measure  of  protection  which  is  afforded  is 
determined  to  a  considerable  extent  by  the  investment  which 
can  be  made  in  protective  apparatus  to  supply  the  protec- 
tion.    In  this  respect  the  cost  of  protection  against  lightning 
is  similar  to  the  cost  of  protection  against  fire  or  accident.     We 
will,  therefore,  in  what  follows,  refer  to  lightning  protectors 
rather  than  to  lightning  arresters.     The  general  term  "light- 
ning protection  equipment"  applies  not  only  to  lightning  pro- 
tectors, but  also  to  other  allied  devices  which  serve  to  minimize 
lightning  damage  to  electrical  apparatus. 

233.  The  Term  Lightning  Has  a  Specific  Significance  when 
used    in    connection    with    electrical    apparatus    protection. 
When  thus  used  "lightning"  implies  to  any  sort  of  excessively- 
high -voltage  disturbance  in  an  electrical  generation,  trans- 
mission or  distribution  system. 

NOTE. — Commercial  lightning  protectors  are  designed  to  protect  only 
against  the  effects  of  transient  abnormal  voltages.  They  are  not,  as  a 
rule,  designed  to  protect  against  the  effects  of  continued  abnormally  high 
voltages,  regardless  of  how  much  such  high  voltages  may  originate. 

234.  Lightning  May  Be  Divided  Into  Two  General  Classes, 

atmospheric  lightning  and  internal  lightning.     Each  of  these 
is  defined  in  following  articles. 

195 


196 


CENTRAL  STATIONS 


[ART.  235 


235.  Atmospheric  Lightning  is  that  due  to  the  equalization 
of  a  difference  of  potential  between  two  oppositely  electrified 
clouds    or  between  a  cloud  and  the  earth.     The  lightning 
strokes  or  lightning  flashes  with  which  everyone  is  familiar 
are  manifestations  of  those  phenomena. 

236.  Atmospheric    Lightning    May    Effect    an    Electrical 
System  in  Either  of  Two  Ways — by  a  so-called  direct  stroke 
or  by  an  induced  stroke. 

237.  A  Direct  Stroke  is  one  where  a  lightning-discharge 

current  between  a  cloud  and 
the  earth  selects,  for  a  portion 
of  its  path,  a  part  of  the  electrical 
system.  Then  that  system  is 
said  to  be  "struck  by  a  direct 
stroke."  Lightning  protectors 
are  not,  as  a  rule,  capable  of 
affording  absolute  protection 
against  direct  strokes.  If  the 
current  of  a  direct  stroke  passes 
through  a  lightning  protector, 
usually  that  protector  is  de- 
stroyed. Direct  strokes  or- 
dinarily strike  only  aerial  pole 
lines.  An  overhead  ground 
wire  strung  (Fig.  150)  above,  or 
adjacent  to,  the  pole  line  and 
connected  with  the  earth  at 
frequent  intervals  affords  the 
most  effective  protection  against 
direct  strokes.  Observation  has 
indicated,  that  the  current 

of  a  direct  lightning  stroke  will  not  flow  along  a  transmission 
line  for  a  very  great  distance.  It  will  usually  find,  through 
some  insulation  breakdown,  a  path  to  earth  a  relatively  few 
feet  away  from  the  point  where  it  "struck"  the  line. 

238.  An  Induced  Stroke  is  one  whereby  an  abnormally 
high  potential  is  developed  on  the  electrical  system,  due  to 
induction,  by  an  atmospheric  lightning  discharge.     Protec- 


FIQ.  150. — Ground  wire  over  a 
three-phase  transmission  circuit. 
(The  line  wires  take  the  positions 
1,  2  and  3.) 


SEC.  11]        LIGHTNING  PROTECTION  APPARATUS 


197 


tion  against  the  effects  of  induced  strokes  is  usually  satis- 
factorily provided  by  a  suitable  lightning  protection  apparatus 
such  as  will  be  described.  The  induced  are  much  more  common 
than  the  direct  strokes. 

239.  Internal  Lightning,  so-called,  is  any  abnormal  voltage 
rise  due  to  changes  in  the  load  on  the  electrical  system.     Ex- 
amples of  internal  lightning  are  the  abnormal  conditions  due 
to  excessively  high  voltages  which  may  occur  particularly  in 
high-voltage  systems  and  which  are  caused  by  the  opening 
or  closing  of  switches  or  by  an  intermittent  ground.     Internal 
lightning  effects  are  sometimes  called  surges. 

240.  A  Lightning  Protector  Is  an  Electrical  Safety  Valve.— 
The  duty  of  the  protector  on  an  electrical  system  is  to  relieve 
the  system  of  abnormally  high  voltages,  in  a  manner  some- 
what analagous  to  that  in  which  a  safety  valve  relieves  a 
steam  boiler  of  an  excessively  high  pressure.     Just  as  the 
safety  valve  should  stop  the 

escape  of  steam  after  the 
abnormal  conditions  have 
been  relieved,  so  should  a 
lightning  protector  stop  the 
flow  of  current  after  the  high 
potential  has  been  relieved. 
Thus,  any  device  which  will, 
under  the  influence  of  a  voltage 
above  normal,  permit  current 
to  flow  through  it  and  which 
will,  when  the  abnormal  con- 


Fio. 


151. — The     principle     of 
lightning  protector. 

dition    ceases  to    exist    stop 

the  flow  of  that  current,  constitutes  a  lightning  protector. 
241.  How  a  Lightning  Protector  Protects  may  be  understood 
from  a  consideration  of  the  diagram  of  Fig.  151.  Assume  that, 
due  to  some  cause  or  other,  the  potential  of  the  line  L  becomes 
much  higher  than  that  of  the  earth  or  ground.  That  is,  as- 
sume that  an  abnormally  high  voltage,  V,  exists  between  the 
conductor  L  and  the  earth  underneath  it.  Such  a  high  voltage 
might  originate  either  from  atmospheric  or  internal  lightning, 
as  above  described.  The  tendency  of  this  voltage  would  be 


198  CENTRAL  STATIONS  [ART.  241 

to  force  a  current  from  L  to  the  earth.  This  current  would 
select  the  path  of  least  opposition.  If  there  were  no  protection 
apparatus  associated  with  the  line,  the  path  of  least  opposition 
to  ground  would,  probably,  be  through  the  windings  of  the 
generator,  M,  to  its  frame  and  from  thence  to  earth,  A,  as  shown 
by  the  dotted  lines.  That  is,  the  high  voltage  would  break 
down  the  insulation  of  some  of  the  windings  of  M  and  force 
a  current  through  them  to  ground.  This  would  damage  the 
machine  and  might,  possibly,  "burn  it  out."  Now  if  an  air 
gap,  G,  were  connected  between  the  line  wire  and  ground,  as 
shown,  the  path  LGE  through  the  gap  to  ground  would  prob- 
ably offer  much  less  opposition,  to  the  flow  of  the  lightning  dis- 
charge current,  than  would  the  path  LCMA  through  the  gen- 
erator to  ground.  The  reason  for  this  is  that  the  path  through 
the  generator  would  probably  be  one  of  relatively  high  induc- 
tance, whereas  the  path  through  the  air  gap,  G,  to  ground 
would  be  one  of  practically  no  inductance. 


•Ground  Connection 


Pole  Line-' 

-Generating  or  Sub-Station 
^Apparatus  to  be  Protected 

FIG.  152. — Diagram  indicating  typical  arrangement  of  lightning  protection 
equipment. 

NOTE. — Lightning  discharge  currents  are  always  of  high  frequencies  or 
the  equivalent  thereof.  Hence,  a  path  containing  inductance  offers 
great  opposition  to  their  flow.  It  is  a  fact  that*  the  electrical  opposition, 
that  is,  the  impedance  offered  by  an  inductive  circuit  to  the  flow  of  an 
alternating  current  increases  as  the  frequency  of  the  current  increases. 
Therefore,  the  abnormal  potential  on  L  would  probably  be  relieved  by  a 
flow  of  high-frequency  current  through  G  to  the  ground,  E.  The  air 
gap,  G,  is  long  enough  to  prevent  its  break  down  and  a  flow  of  current 
under  normal  conditions.  Fig.  152  will  give  a  better  idea  of  actual  con- 
ditions. The  resistances,  R\  and  Ri,  are  provided  to  limit  the  current 
so  that  after  the  abnormal  voltage  condition  has  been  relieved  the  gen- 
erator, Mi,  would  not  force  current  via  the  path  GiRiR-ffz-  If  these 

*  See  the  author's  PRACTICAL  ELECTRICITY. 


SEC.  11]        LIGHTNING  PROTECTION  APPARATUS  199 

resistances  or  their  equivalent  were  not  provided,  the  generator  might 
continue  to  force  current  via  the  path  shown  even  after  the  abnormal 
voltage  condition  had  been  relieved.  The  reason  is  that  after  an  electric 
arc  has  been  established  across  a  gap,  a  relatively  small  voltage  is  suffi- 
cient to  maintain  it.  A  number  of  different  devices  which  are  used  in 
practice  as  an  equivalent  for  the  spark  gaps  illustrated  in  Figs.  151  and 
152  and  some  schemes  utilized  for  preventing  the  flow  of  current  after 
the  high  voltage  has  been  relieved  are  described  in  succeeding  articles. 
NOTE. — Instead  of  there  being  a  difference  of  potential  between  the 
line,  LjL2  (Fig.  152)  and  the  earth  there  might  be  an  abnormal  difference 
of  potential  or  voltage  (internal  lightning)  between  LiLt.  If  the  gaps 
Gi  and  (?2,  were  not  provided  this  excessive  pressure  might  break  down  the 
insulation  in  M  and  damage  it.  But  with  the  gaps,  G\  and  G2,  in  place, 
the  equalization  current  would  flow  via  GiR\R»Gz  so  that  then  the  machine 
would  not  be  damaged. 

242.  The  Function  of  a  Choke  Coil,  CiC2,  Fig.  152,  in  light- 
ning protection  is  to  increase  the  inductance,  therefore  opposi- 
tion, of  the  circuit  in  which  it  is  inserted.     It  thereby  tends 
to  force  the  "high-frequency"  lightning  current  to  ground 
through  the  lightning  protector.    Commercial  types  of  choke 
coils  will  be  described  in  following  paragraphs.     If  a  surge, 
due  to  external  or  internal  lightning,  travels  along  a  trans- 
mission  line,  it  induces  a  very  high  voltage  in  any  inductive 
winding  which   it  encounters.     Hence,   unless   choke   coils, 
which  are  specially  designed  to  provide  this  inductance,  are 
inserted  between  the  line  and  the  apparatus  (transformers  or 
generators)  the  high  voltage  is  likely  to  be  induced  in  the  turns 
of  the  apparatus  and  cause  an  insulation  breakdown  and  con- 
sequent damage. 

NOTE. — Such  damage  may  be  extremely  serious  if  the  current  circulated 
by  the  generators  on  the  system  follows  the  path  provided  by  the  ab- 
normal voltage.  W>iere  choke  coils  are  inserted  as  shown  in  Figs.  151 
and  152  the  high  voltages  will  be  induced  in  the  ends  of  the  choke  coils. 
Damage  to  the  choke  coils  should  not,  however,  occur,  because  the  coils 
are  specially  designed  to  withstand  these  abnormal  conditions. 

243.  An  Important  Distinction  Between  an  Alternating  Cur- 
rent and  a  Direct  Current  From  a  Protector  Standpoint  is  that 
the  alternating  voltage  decreases  to  zero  twice  in  each  cycle, 
whereas  a  direct  voltage  is  always  in  the  same  direction.     The 
consequence  is  that  the  arc  sustained  by  a  direct-current  gen- 


200 


CENTRAL  STATIONS 


[ART.  224 


erator  through  a  spark  gap  after  the  lightning  discharge  cur- 
rent has  passed  is  more  difficult  to  extinguish  than  the  arc 
similarly  sustained  by  an  alternating-current  generator. 
That  is,  it  is  more  difficult  to  stop  the  flow  of  a  direct  current 
through  the  spark  gap  of  a  lightning  protector  than  it  is  to 
stop  the  flow  of  alternating  current.  How  these  character- 
istics are  recognized  in  the  design  of  lightning  protectors  will 
be  described.  First  the  direct-current  types,  then  the  alter- 
nating will  be  treated. 


FIG.  153. — Illustrating  the  prin- 
ciple of  the  magnetic  blow-out 
lightning  protector. 


FIG. 


154.  —  Protector   on  car 
desirable  arrangement. 


244.  A    Magnetic   Blow-out   Direct-current   Protector    is 

shown  diagrammatically  in  Fig.  153.  The  spark  gap,  G,  is  con- 
nected in  series  with  a  blow-out  coil,  B.  When  the  air  gap, 
G,  is  broken  down  by  an  abnormal  voltage,  current  flows  from 
the  line  via  OGBR  to  the  earth,  E.  The  tendency  is  for  the 
current  impelled  by  the  direct-current  generator  to  continue 
to  flow  across  G.  However,  B  develops  a  magnetic  field  as 
shown.  An  arc  can  not  exist  in  a  sufficiently  strong  magnetic 
field.  The  power-current  (generator-current)  arc  is,  there- 


SEC.  11]        LIGHTNING  PROTECTION  APPARATUS 


201 


fore,  "blown  out"  by  the  field.     The  resistance,  R,  limits  the 
current. 

245.  Lightning  Protectors  on  Electric  Railway  Cars  may  be 
arranged  as  shown  in  Figs.  154  and  155  to  protect  the  appa- 
ratus on  the  car.  Protectors  should  also  be  installed  at  inter- 
vals along  the  trolley  line.*  Note  .that  choke  coils,  C,  con- 
stitute part  of  the  car  equipment. 

,.--Carboruncfutn  Block 


FIG.  155.  Fia.  156. 

FIG.  155. — Protector  on  car,  alternative  arrangement. 

FIG.  156. — Sectional  diagram  of  the  carborundum-block  arrester. 

. — The  connection  shown  in  B  is  not  quite  so  effective  as  the  one  shown  in 
A,  due  to  the  greater  length  of  wire  on  the  protector  circuit.  If  it  seems  necessary  to 
use  the  connection  of  B  the  arrester  may  be  placed  on  the  roof  of  the  car,  in  the  vestibule 
or  under  the  car,  without  affecting  the  inductance  of  the  circuit  of  the  arrester.  When 
such  a  connection  is  used,  however,  a  larger  choke  coil  than  in  A  is  necessary  to  offset 
the  greater  inductance  of  the  arrester  circuit). 

246.  A  Carborundum  Block  Protector,  which  may  be  used 
on  either  direct  or  alternating-current  circuits  operating  at 
pressures  not  exceeding  750  volts,  is  shown  in  Fig.  156.  This 
has  been  designated  by  its  manufacturer  J  as  a  multipath  pro- 
tector, because  of  the  fact  that  there  are  many  paths  provided 
to  ground  for  the  lightning  discharge  current.  It  consists  of  a 
disc  or  block,  B,  of  carborundum  granules  bound  together  with 

•  See  the  author's  WIRING  FOE  LIGHT  AND  POWER. 

t  General  Electric  Company. 

J  Westinghouse  Electric  &  Manufacturing  Company. 


202 


CENTRAL  STATIONS 


[ART.  247 


a  dielectric  binding  compound.  On  either  face  of  the  pro- 
tector is  mounted  a  metal  terminal  plate,  PI  and  P2.  A  small 
gap,  G,  is  provided  for  line  voltages  of  from  400  to  750.  The 
two  terminals,  E  and  L,  are  connected  between  ground  and 
line  respectively.  When  under  the  influence  of  an  abnormally 
high  voltage  the  dielectric  is  broken  down,  the  protector  oper- 
ates permitting  current  to  flow.  When  the  abnormal  voltage 
is  equalized  the  current  flowing  through  the  block  ceases  be- 
cause the  many  minute  electric  arcs  through  the  block  can  not 
be  maintained  by  the  generator  pressure. 

247.  The  Condenser-type  Protector  for  direct-current  cir- 

cuits is  diagrammed  in  Fig. 
157.  These  are  designed  par- 
ticularly for  circuits  operating 
at  pressures  of  from  750  to 
1 , 500  volts.  It  consists  merely 
of  a  spark  gap,  G,  in  series 
with  a  resistor,  R.  However, 
the  resistor  is  shunted  by  a 
permittor  (condenser).  The 
spark  gap  prevents  the  flow 
of  current  through  the  pro- 
tector when  the  voltage  is 
normal.  When  the  voltage 

becomes  abnormal  current  is  forced  across  the  gap  and 
the  permittor  permits  free  discharge  of  the  high-frequency 
lightning  current  to  ground.  The  direct  generator  current 
can  not  flow  because  it  can  not  pass  through  the  permittor  and 
the  resistance  of  R  is  so  great  that  even  if  G  is  short-circuited 
the  direct  current  which  will  flow  is  negligibly  small.  The  real 
function  of  R  is  to  maintain  the  permittor  in  a  discharged  con- 
dition. Protectors  of  this  general  type  but  without  the  spark 
gap  are  also  manufactured  and  are  recommended  for  the  pro- 
tection of  apparatus  having  weakened  insulation. 

248.  The  "Circuit-breaker"  Type  Lightning  Protector  is 
shown  diagrammatically  in  Fig.  158.     This  design  comprises 
essentially  four  air  gaps,  Oit  Gz,  Gs,  and  (?4,  with  a  resistance 
between  them.    The  assembled  device  is  shown  in  Fig.  159. 


FIG.  157. — The  condenser-type  pro- 
tector. 


SEC.  11]        LIGHTNING  PROTECTION  APPARATUS 


208 


The  two  gaps,  G3(?4,  on  the  ground  side  of  the  current- 
limiting  resistor,  have  connected  in  multiple  around  them  a 
solenoid,  S  (Fig.  158).  When  a  current  of  sufficient  intensity 
passes  through  S,  the  iron  plunger,  P,  is  lifted  by  the  magnetic 
effect  thereof  and  then  opens  the  circuit  at  B.  It  is  due  to 


S  Insulating  Base  From\Lw 


£>.  Q: 


Resistance--*  M 


FIG.  158. — The  diagram  of  "circuit- 
breaker-type"  lightning  protector. 


•Fiber  Tube 


Fio.  159. — The    Carton-Daniels 
lightning  protector. 


this  device  that  the  protector  is  called  the  "circuit-breaker" 
type.  When  the  protector  is  discharging  the  high-frequency 
current  of  a  line  at  abnormal  voltage  practically  all  of  this 
current  passes  directly  through  the  path  LiGiG^GsGJ^z  to 
ground.  The  inductance  of  the  solenoid  S  is  so  high  that  prac- 
tically none  of  the  high-frequency  current  will  go  through  it. 
However,  if  the  power  current  follows  the  lightning  discharge 
current  through  the  protector,  this  power  current  will  not, 
because  of  its  low  frequency,  pass  across  gaps  G3(?4  but  will,  in 


204 


CENTRAL  STATIONS 


[ART.  249 


preference,  take  the  path  of  lesser  opposition  through  S. 
Thereby  the  plunger  is  raised  and  the  arc  extinguished. 

249.  Non-arcing  Metal  Cylinder  Protectors  (Figs.  160  and 
161)  may  be  used  on  alternating-current  circuits  operating  at 
pressures  below  300  volts.  The  metal  cylinders  are  usually 


,.-  Cast  Iron  Case 


FIG.  160. — Illustrating  the  prin- 
ciple of  the  non-arcing  metal  gap 
protector. 


I- Protector  Element 
FIG.  161. — Double-pole,  non-arcing 
metal  cylinder  protector. 


composed  of  a  copper-zinc  alloy.  In  an  alternating-current 
circuit,  an  arc,  established  by  a  lightning  voltage,  will  not  be 
maintained  between  these  cylinders  by  the  power  current. 
The  reason  is  that  the  arc  will  extinguish  when  the  power 
alternating-current  wave  passes  through  zero.  The  zinc  in 
the  alloy  vaporizes  at  a  relatively  low  temperature  and  this 
vapor  tends  to  quench  the  arc.  There  is  also  a  rectifying 
phenomena  in  a  protector  of  this  character.  The  metallic 


SEC.  11]        LIGHTNING  PROTECTION  APPARATUS 


205 


Cast  Iron  Cafe 


vapor  conducts  the  current  in  one  direction  only.  Therefore, 
when  the  current  reverses,  the  metallic  vapors  tend  to  cool 
below  the  arcing  temperature  before  the  alternating-current 
wave  is  completed.  Thereby,  the  arc  may  be  extinguished. 

250.  Protectors  Comprising  Non-arcing  Metal  Caps  with 
a  Resistance  in  Series  may  be  used  for  alternating-current 

circuits  operating  at  pres- 
sures below  3,000  volts.  A 
protector  of  this  type  (Fig. 
162)  comprises  a  sufficient 
number  of  spark  gaps  to 
prevent  the  passage  of  cur- 
rent at  normal  voltage.  In 
series  with  the  gaps  is  ar- 
ranged a  resistor  to  limit 
the  dynamic  (generator)  cur- 
rent which  tends  to  flow  to 


FIG.  162. — Multigap  alternating- 
current  arrester  with  series  resistance 
arranged  for  pole  mounting. 


FIG.  163. — Multigap  protectors 
mounted  on  a  pole  for  trans- 
former protection. 


ground  after  an  arc  has  been  established  through  the  gaps 
by  a  lightning  discharge.  Fig.  163  shows  how  three  pro- 
tectors, P\,  Pz  and  P3,  of  this  type  may  be  mounted  on  a  pole 
for  transformer  protection.  LI,  L2  and  Ls  are  the  line  wires. 
251.  The  Graded-shunt  Resistance  Protector,  the  principle 
of  which  is  illustrated  in  Fig.  164,  consists  of  a  series  of  spark 
gaps  (Gi  to  Cru)  associated  with  resistors  which  are  shunted 
around  them.  Protectors  of  this  general  type  are  manufac- 
tured for  voltages  of  from  1,200  to  13,000.  That  shown  in 


206 


CENTRAL  STATIONS 


[ART.  251 


Fig.  164  is  for  2,200  volts.  There  are  three  alternate  paths 
for  discharge  through  the  protector  of  Fig.  164.  One  path 
comprises  all  of  the  gaps,  G\  to  (ru,  in  series.  Another  path 
comprises  four  gaps  (Gfi  to  (r4)  in  series  with  a  relatively  low 
resistance  R i.  The  other  path  comprises  two  gaps,  GI  and 
G2,  in  series  with  a  high  resistance  R2.  Three  paths  to  ground, 
each  of  a  different  impedance,  are  thus  offered  through  the 
protector.  Thereby  a  discharge  of  any  frequency  will  find  a 


Insulating 


FIG.  164. — Diagram    of    a    so-called 
"multipath"  lightning  arrester. 


FIG.  165. —  Multipath    protector  in 
wooden  box  for  outdoor  installation. 


path  of  a  relatively  low  impedance  for  it  to  ground.  The  path 
comprising  the  high  resistance  and  the  two  series  gaps  is  for 
discharges  of  low  frequency  or  for  discharging  gradual  accu- 
mulation of  static  electricity.  Discharges  of  medium  frequen- 
cies will  select  the  path  comprising  the  low  resistance  and  the 
four  series  gaps.  Discharges  of  high  frequency  will  select  the 
third  path  which  includes  only  the  gaps  in  series. 

NOTE. — Why  the  lightning  takes  different  paths  may  be  explained 
thus:*  When  the  gaps  of  a  protector  are  shunted  by  a  low  resistance  a 
discharge  of  the  high  frequencies  finds  it  relatively  difficult  to  pass 
through  the  resistance  rods.  This  is  because  of  the  impedance  of  the 

•  General  Electric  Company. 


SEC.  11]         LIGHTNING  PROTECTION  APPARATUS 


207 


rods.  However,  such  a  discharge  will  follow  with  relative  ease  across  all 
of  the  gaps  because  of  the  permittive  (electrostatic  capacity)  effect  of 
the  gaps.  The  series  of  gaps  is,  in  effect,  a  number  of  permittors  (con- 
densers) in  series.  The  higher  the 
frequency  the  more  pronounced  is 
this  effect.  Hence,  the  discharges 
select  the  paths  through  the  gaps  and 
resistances  which  offer  the  least  opposi- 
tion. Which  path  is  selected  in  any 
case  will  be  determined  by  the  fre- 
quency of  the  lightning  discharge.  By 
frequency  is  meant  not  the  generator 
frequency  but  the  equivalent  lightning 
frequency  which  may  be  hundreds  of 
thousands,  or  even  millions,  of  cycles 
per  second.  If  the  power  current 
tends  to  follow  a  lightning  discharge 
it  will ,  because  of  its  relatively  low 
frequency,  select  one  of  the  paths 
through  a  resistance  which  will  limit  it. 
Due  to  the  rectifying  effect  (described 
above)  of  the  spark-gap  cylinders,  the 
arc  should  be  extinguished  at  the  end 
of  the  first  K  cycle  after  the  generator 
current  has  started  to  flow.  Fig.  165 
shows  one  of  these  protectors  arranged  in  a  wooden  box  for  outdoor  work 
installation.  G  is  the  ground  wire  and  L  is  the  line  wire. 


l-lndoor  Tape  JL-Outdoor  Type 

FIG.  166. — The  "  cupped-disc- 
gap"  protector.  (Westinghouse 
Elec.  Mfg.'  Company.) 


'-•Ground  Wire 
FIQ.  167. — Outdoor-type,  cupped-disc-gap  protectors  on  a  three-phase  line. 


208 


CENTRAL  STATIONS 


[ART.  252 


252.  A  Cupped-disc  Gap  Protector  is  delineated  in  Figs.  166 
and  167.  This  type  of  equipment  may  be  designed  for  alter- 
nating-current pressures  of  from  3,000  to  13,000  volts.  It 
comprises  (Fig.  166)  a  series  of  cup-shaped  discs,  D,  of  a  non- 
arcing  metal  supported  on  an  insulating  rod,  together  with  a 
resistance,  R,  in  series.  The  line  wire  is  connected  to  terminal, 
L,  and  the  ground  wire  to  G.  This  form  was  designed  specially 
for  distributing-transformer  protection  and  in  practice  is  ar- 
ranged on  the  pole  line  as  shown  in  Fig.  167  by  suspending  it 
from  the  line  wire  with  a  porcelain  insulator.  Pi,  P2  and  P3 
are  the  protectors. 


FIG.  168. — Horn-gap  protector  for  A.  C.  series  lightning  circuits. 

253.  Horn-gap  Protectors  are  shown  in  Figs.  168  and  169. 
An  electric  arc  once  established,  due  to  some  cause  or  other, 
between  two  horns  tends  to  extinguish  itself.  The  arc  rises 
on  the  horns,  due  to  the  upward  flow  of  the  column  of  hot 
gases  and  finally  attains  a  length  of  gap  which  it  cannot  main- 
tain. Fig.  168  shows  a  protector  of  this  type  for  use  on  alter- 
nating-current series  street  lighting  circuits  where  the  normal 
pressure  does  not  exceed  1,500  volts.  Z/i  and  L2  are  the  line 


SEC.  Ill         LIGHTNING  PROTECTION  APPARATUS 


209 


Porcelain 

Insulator-" 

Split  Sleeve  and- 
Screws  which 
Clamp  Protector 
to  Line  Wire 


•Iron  Pipe  Full 
Length  of 

Insulator 


FIG.  169. — Horn-gap,  choke-coil  pro-  FIG.  170. — The    Pierce    line-wire, 

tector  for  alternating-current  lines.        horn-gap  protector.    (The  iron  pipe 

acts  as  a  choke  coil.) 


Metal  Cap* 


Compound 


Line- 


Current-Limiting-  •  •  •> 
Resistor 


Porcelain 
Separators 


FIG.  171. — The  Burke  suspension 
lightning  protector  for  pressures  up 
to  6600  volts. 


,  Section  A-k 


FIG.  172.— The  General  Electric 
Company  compression-type  lightning 
protector. 


210  CENTRAL  STATIONS  [ART.  254 

wires  and  E  is  the  ground  connection.  Protectors  of  this  same 
general  form  are  manufactured  for  alternating  pressures  up 
to  9,000  volts.  Abnormal  voltage  conditions,  due  to  surges, 
are  common  on  alternating-current  series  lighting  circuits. 
Such  surges  may  be  caused  by  the  opening,  short-circuiting 
or  grounding  of  the  circuits.  These  simple  horn-gap  protec- 
tors appear,  for  this  service,  to  provide  effective  protection. 
Electrolytic  protectors,  the  principle  of  which  will  be  described 
later,  are  preferable  for  the  protection  of  direct-current  series 
lighting  circuits.  Figs.  170  and  171  show  recently  developed 
types  of  horn-gap  protectors  arranged  for  suspending  on  line 
wires.  In  both  of  these  illustrations:  LI  is  the  line- wire  con- 
nection; L2  leads  to  the  apparatus  (or  choke  coil  if  there  is 
such);  and  G  is  the  ground  wire.  A  current-limiting  resistor 
R  is  provided  in  the  protector  of  Fig.  171. 

254.  A  Combination  Choke  Coil  and  Horn-gap  Protector  is 
shown  in  Fig.  169.     These  protectors  are  designed  for  alter- 
nating-current voltages  of  33,000  and  above.     If  an  arc  is 
established  across  the  gap  G  it  tends  to  rise  and  extinguish 
itself  as  above  described.     And,  furthermore,  the  choke  coil 
produces  a  magnetic  field  which,  because  of  the  phenomena 
described  above  in  connection  with  a  magnetic  blowout  pro- 
tector, assist  in  the  rapid  quenching  of  the  arc. 

255.  The  Compression-type  Protector  (Fig.  172)  comprises 
a  number  of  gaps  arranged  in  series  with  a  resistor  inside  of  a 
closed  porcelain  tube.     An  iron  tube  which  is  grounded,  sur- 
rounds the  air  gaps  and  equalizes  the  electrostatic  gradient. 
When  a  discharge  passes  between  the  metal  electrodes  which 
form  the  gaps,  it  expands  the  air  and  compresses  it,  thus  ex- 
tinguishing the  arc.     Protectors  of  this  type  are  most  fre- 
quently used  on  2,200-volt  pole  lines  for  distributing-trans- 
former protection.     They  have  a  limited  discharge  capacity. 

256.  The  Electrolytic  Lightning  Protector,  provides,  prob- 
ably, the  most  effective  insurance  against  lightning  damage 
now  known.     Protectors  of  this  type  can  be  furnished  for 
alternating-current  and  direct-current  circuits  of  any  com- 
mercial  voltage.     Their   disadvantages   are   that   they   are 
relatively  expensive  and  that  they  require  a  certain  amount 


SBC.  11]         LIGHTNING  PROTECTION  APPARATUS 


211 


of  attention.  However,  these  disadvantages  are  of  minor 
consequence  when  expensive  equipment  is  to  be  protected. 
Only  the  alternating-current 
protectors  will  be  described 
here. 

257.  The  Principle  of  the 
Electrolytic  Protector  may  be 
understood  from  a  consideration 
of  Figs.  173  and  174.  The 
protector  consists  of  a  stack 
(Fig.  173)  of  cone-shaped, 
aluminum  plates  or  trays  spaced 
about  0.3  in.  apart.  A  solution 
of  electrolyte  is  poured  into  the 
spaces  between  the  plates.  The 
completed  stack  is  mounted  in  an 
iron  tank  which  is  then  filled 
with  oil.  The  oil  not  only  pre- 
vents the  evaporation  of  the 
electrolyte  but  also  prevents  a 
rapid  rise  in  temperature  when 
the  protector  is  discharging. 
The  upper  plate  of  the  stack 
is  connected  to  a  horn  gap  (Fig. 
175).  The  lower  plate  is  also 
usually  grounded  on  the  tank  which  is  also  (on  a  grounded 
neutral  system)  connected  to  ground. 


FIG.  173. — Sectional  elevation 
of  a  General  Electric  Company 
electrolytic  protector. 


Ampere* 

Flo.  174. — Graph  showing  now  the  electrolytic  protector  permits  current 
to  flow  readily  at  pressures  above  330  volts  per  cell. 


212  CENTRAL  STATIONS  [ART.  258 

258.  The  Chemical  Action  of  the  Electrolyte  usually  is 
such  that  it  forms  on  the  surfaces  of  the  aluminum  plates  a 
film  of  hydroxide  of  aluminum.  At  voltages  below  about 
350  (Fig.  174)  this  film  has  an  exceedingly  high  resistance. 
However,  at  voltages  in  excess  of  350  the  resistance  of  the 
film  is  very  small.  Thus,  an  electrolytic  cell  arranged  as  sug- 
gested, forms  an  electrical  safety  valve  which  operates  at  a 
pressure  of  approximately  350  volts.  However,  at  voltages 
below  350  some  current  would  flow  through  the  protector  if  it 
were  left  connected  to  a  "line"  circuit  wire.  Therefore,  it  is 


Fia.  175. — Schematic  connection    diagram    for  a  grounded-neutral,    three- 
phase  electrolytic  protector. 

necessary  to  connect  in  series  with  it  a  horn  gap,  as  shown  in 
Fig.  175.  The  number  of  aluminum  plates  which  is  neces- 
sary to  connect  it  in  series  is  determined  by  the  normal 
voltage  of  the  line  to  be  protected.  There  should  be  one  cell, 
approximately,  for  each  350  volts  of  normal  line  pressure. 
Fig.  176  shows  the  construction  of  an  electrolytic  protector 
for  150,000  volts.  Fig.  177  shows  a  complete  installation. 

NOTE. — For  charging  an  electrolytic  protector  the  horn  gaps  are  closed 
together  by  moving  a  suitably  arranged  lever.  One  side  of  the  horn  gap 
is  hinged  to  provide  for  this. 

259.  The  Arrangement  of  Electrolytic  Protectors  on 
Grounded  and  Ungrounded  Neutral  Three-phase  Systems 
is  shown  respectively  in  Figs.  175  and  178.  Where  the  neutral 


SEC.  11]        LIGHTNING  PROTECTION  APPARATUS 


213 


is  grounded,  a  voltage  greater  than  the  normal  voltage  be- 
tween phase  wires  (Li,  L2  and  L3)  can  never  be  impressed  across 
any  one  of  the  three  cells  (1,  2,  and  3)  if  they  are  arranged  as 


Electrolyte 


FIG.  176. — Section  through  a  general-electric-company  11 5,000- 135,000- volt 
electrolytic  arrester  tank. 

shown  in. Fig.  175.  However,  if  the  neutral  is  ungrounded 
and  an  accidental  ground  occurs  some  place  on  the  line,  a 
voltage  equal  to  almost  twice  normal  voltage  (1.73  X  normal 


214 


CENTRAL  STATIONS 


[ART.  259 


FIG.  177. — A    Westinghouse     Electrolytic     protector     for     a     three-phs 
ungrounded  neutral  22,000- volt  circuit. 


Pmim^^M&mmm 
^/^r^^yy^^i^^ff^w^ 


FIQ.  178. — Schematic    diagram    for    an    ungrounded    neutral    three-phase 
electrolytic  protector. 


SBC.  11]        LIGHTNING  PROTECTION  APPARATUS 


215 


voltage)  would  be  impressed  across  one  of  the  protector  tanks 
if  only  three  were  used  (Fig.  175).  Therefore,  where  the 
neutral  is  ungrounded,  a  fourth  cell  or  tank  (4)  is  placed  in 
the  ground  lead,  as  diagrammed  in  Fig.  178,  to  insure  that  a 
voltage  greater  than  normal  will  never  be  impressed  across 
any  one  of  the  four  cells.  E  is  the  ground  connection. 

260.  In  Selecting  Choke  Coils  it  is  necessary  to  exercise 
judgment.  In  a  general  way  the  protective  ability  of  a  choke 
coil  increases  as  the  square  of  the  mean  diameter  of  the  coil. 
With  choke  coils  of  equal  length  and  equal  mean  diameter* 


•Terminal 

* «#' 

FIQ.  179. — 100-amp.  choke  coil  designed  for  pressures  of  6600  volts  and  lower. 
(General  Electric  Company.) 

the  protective  ability  varies  as  the  square  of  the  number  of 
turns.  From  the  standpoint  of  lightning  protection  a  large 
choke  coil  is  desirable.  However,  the  larger  the  coil  the 
greater  its  impedance  and  resistance.  If  a  coil  is  too  large 
the  voltage  drop  and  energy  loss  in  it  will  be  excessive,  hence 
in  selecting  the  coil  it  is  desirable  to  consider  these  features 
and  choose  one  of  a  size  which  practice  has  shown  to  provide 
sufficient  protection  without  excessive  energy  loss  or  voltage 
drop. 

261.  A  Choke  Coil  for  Low-voltage  Circuits  is  shown  in 
Fig.  179.  It  comprises  merely  a  coil  of  insulated  wire  of 
sufficient  cross-sectional  area  to  carry  the  current  of  the  cir- 
cuit into  which  it  is  to  be  connected.  This  coil  is  wound  on 
an  insulating  core  which  is  mounted  on  an  insulating  base. 

•  Westinghouse  Electric  and  Manufacturing  Company. 


216 


CENTRAL  STATIONS 


[ART.  262 


Suitable  terminals  are  provided.  The  core  is  not  necessary 
except  to  insure  mechanical  rigidity.  Home-made  choke  coils 
can  be  readily  constructed  by  forming  a  helix  of  wire. 

262.  Air-insulated  Choke  Coils  for  higher  voltages  are  con- 
structed as  suggested  in  Figs.  180,  181  and  182.  The  type 
shown  in  Fig.  180  offers  very  effective  protection,  but  is  ex- 


Insulated- 

Strap-Copper 

Winding 


FIG.  ISO. — Westinghouse  "pan- 
cake type  "  Choke  coil  for  pressures 
of  from  2200  to  25,000  volts. 


FIG.  181. — General  Electric  Com- 
pany "hour-glass"  choke  coil  in- 
sulated for  35,000  volts. 


pensive  because  of  the  large  amount  of  copper  involved  in  its 
construction.  Hence,  coils  of  the  general  design  of  Figs. 
181  and  182  (A  is  the  apparatus  terminal  and  L  the  line 
terminal)  are  used  more  frequently,  particularly  on  very  high- 
voltage  lines  for  which  the  construction  of  Fig.  180  would 
not  be  suitable. 

263.  Oil-insulated  Choke  Coils  are  sometimes  used  on 
high-voltage  alternating-current  lines  and  comprise  merely 
(Fig.  183)  a  coil  immersed  in  a  suitably-insulated  and  designed 


SEC.  11]        LIGHTNING  PROTECTION  APPARATUS 


217 


steel  tank  which  is  filled  with  oil.  The  oil  insulates  the  coil 
against  side  flashes  and  dissipates  the  heat  developed  in  it 
so  that  a  conductor  of  small  cross-sectional  area  can  be  used 
for  the  coil. 


FIQ.     182. — Air-insulated     choke    coil 
pressures  up  to  150,000  volts. 


for  FIG.    183. — Westinghouse    oil- 

insulated,  self-cooling  choke  coil 
for  pressures  of  from  25,000  to 
70,000  volts. 


264.  Application  of  Alternating-current  Lightning  Protec- 
tors.— The  following  table  indicates  in  a  general  way  the 
services  for  which  certain  of  the  Westinghouse  protectors  of 
the  different  types  are  fitted.  The  price  increases  from  the 
top  to  the  bottom  of  the  table. 


218 


CENTRAL  STATIONS 


[ART.  264 


! 


If 

STJ 


I 


II 


III! 


and  for 
protecti 


1 


il 

f 

58 


O  "" 

i 

in 
O 

S 


fl 

II 


<•§ 


111 

£-~£ 

sll 

HI 


Greater 
equivale 
or  extra 


K*ta 

H?"Ei 


ii  si 


SECTION  12 


AUTOMATIC  VOLTAGE  REGULATORS 

265.  The  Desirability  of  Maintaining  Constant  the  Voltage 
Impressed  by  a  Generator  is  well  recognized.     This  is  par- 
ticularly true  where  an  incandescent  lamp  load  is  served  by 
the  generator.     The  graph  of  Fig.  184  shows  that  a  small  de- 
crease in  voltage  results  in  a  material  decrease  of  candle- 
power  and  wattage.    A  decrease  in  the  wattage  involves  a 
corresponding  loss  in  revenue 

to  the  central  station.  A  de- 
crease in  candle-power  in- 
volves dissatisfaction  of  the 
consumer. 

EXAMPLE. — Referring  to  the 
graph  of  Fig.  184,  a  2  per  cent, 
decrease  in  voltage  decreases  the 
candle-power  to  93}£  per  cent, 
of  the  normal  candle-power  and 
the  wattage  to  96>£  per  cent,  of 
the  normal.  Furthermore,  fewer 
lamp  renewals  are  necessary  where 
the  voltage  impressed  on  the 
lamps  is  maintained  constant  and 
also  higher-efficiency  lamps  may 

be  used.  When  the  voltage  increases  above  normal,  the  lives  of  the 
lamps  are  correspondingly  decreased.  While  for  the  operation  of  motors 
it  is  not  so  essential  that  the  voltage  variation  be  a  minimum,  it  is  de- 
sirable because  burn-outs  of  motors  and  control  apparatus  may  result 
if  the  voltage  is  too  low. 

266.  There   are    Several   Factors  Which  Tend  to  Cause 
Variations  in  the  Voltage  impressed  by  the  generator  on  the 
bus-bars.     The  prime-mover  speed  may  not  be  constant — 
this  holds  true  for  both  steam  prime  movers  and  waterwheels. 
Voltage  variation  can  also  be  due  to  the  /  X  R  drop  in  a  gen- 

219 


86      92       9fc"K»""l04™'l08"yll21 
Fio.    184. — Graph  showing    varia- 
tion of  candle  power  and  wattage  of 
tungsten  lamps  with  variation  in  im- 
pressed voltage. 


220 


CENTRAL  STATIONS 


[ART.  267 


erator  which  increases  with  the  load.  With  alternating-cur- 
rent machines,  variation  in  generator  voltage  will  result  when 
the  exciter  voltage  varies  due  to  some  cause  or  other. 

267.  The  Function  of  the  Automatic  Voltage  Regulator  is  to 
maintain  constant  the  voltage  which  is  impressed  by  a  gen- 
erator on  the  bus-bars.  This  function  is  performed  accurately 
and  most  satisfactorily  by  automatic  regulators  of  the  Tirrill 
type,  the  principles  of  which  will  be  described  in  following 
articles. 


Relay  Contact-.. 


Condenser  to 
Minimize 
Arcing  at  A 


.Differentially-  Wound  Magnet 


Spring 


FIG.  185. — Arrangement   of   an    automatic   voltage   regulator    for   a    small 
direct-current  generator. 

268.  The  Principle  of  the  Automatic  Voltage  Regulator  is 
illustrated  in  Fig.  185.  The  voltage  impressed  by  any  gen- 
erator on  its  bus-bars  can  be  maintained  almost  constant  by  a 
man  operating  the  field  rheostat.  However,  such  a  method 
would  be  very  expensive  and  would  not  effect  as  close  voltage 
regulation  as  will  the  automatic  device  to  be  described.  The 
principle  of  the  automatic  regulator  is  this:  If  the  voltage 
impressed  by  the  generators  on  the  bus-bars  increases,  the 
automatic  regulator  places  a  shunt  circuit  around  (or  short- 
circuits)  the  field  rheostat  of  the  machine.  This  permits  a 
greater  field  current  to  flow  and  the  generator  voltage  then 


SEC.  12]  AUTOMATIC  VOLTAGE  REGULATORS  221 

increases.  When  the  generator  voltage  has  attained  normal 
the  short-circuit  around  the  field  rheostat  is  removed  and  the 
voltage  then  tends  to  decrease.  In  an  actual  regulator  con- 
trolling a  generator  serving  a  varying  load,  this  short-circuit 
is  continually  being  placed  around  the  rheostat  or  moved 
therefrom,  as  occasion  demands.  The  result  is  that  the  con- 
tacts which  make  and  break  the  short-circuit  path  are  moving 
continually  somewhat  as  the  contacts  in  an  electric  vibrating 
bell  move.  However,  the  vibration  of  the  automatic  regulator 
contact  is  not  uniform  because  under  certain  constant  load 
conditions,  the  contactor  may  not  vibrate  at  all.  Obviously, 
then,  the  regulation  depends  on  the  rapid  making  and  breaking 
of  the  short-circuit  in  contacts. 

EXPLANATION. — The  arrangement  of  Fig.  185,  showing  a  voltage  reg- 
ulator for  a  small  direct-current  generator,  is  designed  to  maintain  a 
constant  voltage,  EL,  across  bus-bars  -LiL2.  The  closing  of  the  contact  A 
short-circuits  the  field  rheostat,  R.  The  opening  and  closing  of  contact 
A  is  in  turn  controlled  by  the  differential  magnet  M.  Magnet  M  has 
two  opposing  windings,  W\  and  Wz.  One  of  these  windings  is  in  series 
with  contact  C,  the  opening  and  closing  of  which  is  controlled  by  relay 
B  which  is  connected  across  the  bus-bars.  When  C  is  opened  only  wind- 
ing WL  is  excited.  A  is  then  opened  by  the  pull  of  Wi.  When  contact 
C  is  closed,  W2  is  also  excited,  which  neutralizes  the  effect  of  W\.  Then 
A  is  closed  by  the  action  of  the  spring  Si.  Now,  if  the  voltage,  EL,  rises 
above  normal,  relay  B  is  excited  sufficiently  to  overcome  the  pull  of  spring 
52.  B  then  pulls  down  plunger  P  and  opens  contact  C,  deenergizing  Wi. 
Thereby  contact  A  is  opened,  removing  the  short-circuit  path  around 
R  and  inserting  R  in  the  shunt-field  circuit.  The  insertion  of  R  in  the 
field  circuit  decreases  the  field  current  and  excitation  and  decreases  the 
voltage  developed  by  G.  If  the  voltage,  Ei,  decreases,  the  operation  is 
reversed.  The  ultimate  result  is  that  the  contacts  A  and  C  are  in  almost 
constant  vibration.  They  remain  either  open  or  closed  for  such  longer 
or  shorter  intervals  as  may  be  necessary  to  maintain  EL  constant. 

269.  Voltage  Regulators  for  Small  Direct-current  Genera- 
tors operate  on  the  same  principle  illustrated  in  Fig.  185  and 
described  in  the  above  explanation.  The  exterior  appearance 
of  one  of  these  devices  is  shown  in  Fig.  186,  which  is  lettered 
to  correspond  with  the  diagram  of  Fig.  185.  If  the  shunt-field 
current  is  greater  than  can  be  satisfactorily  ruptured  by  one 
contact,  A  (Figs.  185  and  186),  several  of  these  contacts  can 


222 


CENTRAL  STATIONS 


[ART.  270 


be  arranged  in  multiple.  A  multiple-contact  regulator,  oper- 
ating on  the  principle  shown  in  Fig.  185,  can  control  direct- 
current  generators  of  capacities  up  to  125  kw. 


Main  Control  Magnet-' 
FIG.  186. — Regulator  for  small  direct-current  generators. 


FIG.  187. — Illustrating  the  principle  of  an  automatic  voltage  regulator  as 
applied  to  a  small  alternating-current  generator. 

270.  The  Principle  of  the  Alternating-current  Regulator  is 
shown  in  Fig.  187.  This  operates  on  the  shunt-field  circuit 
of  the  exciter,  thereby  controlling  the  alternating-current  gen- 


SBC.  12] 


AUTOMATIC  VOLTAGE  REGULATORS 


223 


erator  field.  Fig.  187  shows  only  the  principle,  because,  as 
will  be  described,  the  actual  construction  of  the  regulator  for 
an  alternating-current  circuit  is  more  complicated  than  that 
suggested  in  Fig.  187. 

EXPLANATION. — If  the  alternating-current  generator  voltage,  EL  (Fig. 
187),  increases  above  normal  the  plunger  in  the  solenoid  or  relay  S  is 
raised.  This  opens  contact  C2.  Then  winding  TF»  is  deenergized  and 
the  pull  of  W\  opens  contact  Ci,  thereby  the  resistance,  R,  is  inserted  in 
the  exciter-field  circuit.  The  voltage  EL  will  decrease,  which  weakens 
the  pull  of  S,  closing  contact  Cz.  Then,  because  the  effect  of  Wz  neutral- 
izes that  of  W\,  spring  S  will  close  C\.  Thereby  R  is  short-circuited  and 
the  exciter-field  excitation  of  GD  is  increased.  Thus,  the  alternating- 
current  generator  voltage  is  again  raised. 


,-„__- '^-Alternating -Current 

-•Generator  Field  Rheostat  '        Generator 
FIG.  188. — Automatic  voltage  regulator  for  large  alternating-current  gen- 
erators.    (This  shows  a  three-phase  generator.) 

271.  The  Actual  Arrangement  of  Voltage  Regulators  for 
Alternators  is  diagrammed  in  Fig.  188  and  their  appearance 
shown  in  Figs.  189  and  190.  Greater  sensitiveness  of  control 
is  effected  with  this  device  than  with  those  of  Figs.  185  and 
187.  Solenoid  A  is  energized  by  a  current  proportional  to  the 
voltage  on  the  bus-bars.  It  is  usually  fed  through  a  potential 
transformer,  B.  The  core  of  A  is  attached  to  a  lever  C.  On 
the  opposite  end  of  C  is  a  contact  D  and  a  balancing  weight  E. 


224 


CENTRAL  STATIONS 


[ART.  271 


When  A  is  energized  C  is  lifted  and  D  is  opened.  The  windings 
A  and  K  (the  function  of  K  will  be  described  later)  constitute 
the  alternating-current  control  magnet.  The  direct-current 
control  magnet  F  is  connected  across  the  exciter  terminals. 


Insulating  Panel-' 


Contacts 


FIG.  189. — Automatic  voltage 
regulator  for  small  capacity  exciters 
mounted  on  a  31-in.  panel  for  in- 
sertion in  a  switchboard. 


FIG.  190. — A  large  capacity  regu- 
lator having  five  relay  contacts. 
(See  Figs.  188  and  191  for  diagram). 


When  F  is  energized  G  is  pulled  down  against  the  tension  of  the 
spring  jSi,  tending  to  open  the  contact  D.  The  pull  on  F  is  in 
direct  proportion  to  the  exciter  voltage.  All  other  features 
are  substantially  similar  to  those  previously  described. 


SEC.  12] 


AUTOMATIC  VOLTAGE  REGULATORS 


225 


EXPLANATION. — If  the  alternating  voltage  decreases  to  below  normal, 
the  plunger  in  A  (Fig.  188)  closes  contact  D.  This  permits  current  to 
flow  via  the  path  shown  by  the  dotted  arrows,  thus  closing  contact  7  and 
short-circuiting  the  exciter-field  rheostat.  Thereby  the  exciter  voltage 
and  the  alternating  voltage  is  raised.  Now  as  the -exciter  voltage  in- 
creases, F  is  energized  and  contact  D  is  raised.  However,  if  the  alter- 
nating voltage  remains  low  the  lower  contact  at  D  follows  the  upper  one. 
If  now  the  alternating  voltage  increases  above  normal  contact  D  is  opened, 
which  again  causes  R  to  be  inserted  in  the  exciter-field  circuit.  Where 


Main  Feetfers* 

.••Contacts    .Pivot       Compensating'  '    ' 

•..-Winding     . 


FIG.  191. — Schematic    diagram    of  connections    for  a  regulator  controlling 
large  exciters,  several  relay  contacts  are  used. 

the  exciter  capacity  is  small  or  where  there  is  only  a  single  exciter  one 
contact  suffices  (Fig.  189)  at  /.  In  stations  where  there  are  a  number  of 
exciters  the  relay  H  operates  a  number  of  contacts,  /.  There  may  be 
one  or  more  than  one  relay  contact  for  each  exciter  as  shown  in  Figs. 
190  and  191.  In  important  installations  a  separate  regulator  may  be 
used  for  each  exciter. 

272.  A  Voltage  Regulator  for  Large  Direct-current  Genera- 
tors operates  on  a  principle  similar  to  that  diagrammed  in 
Fig.  188.     However,  in  the  direct-current  regulator,  the  con- 
tacts must  short-circuit  the  generator-field  winding  because 
there  is  no  exciter  on  which  they  can  act. 

273.  The  Capacity  of  the  Relay  Contacts,  in  amperes,  is 
what,  in  general,  determines  the  capacity  of  the  regulator. 
One  contact  has  a  capacity  of  about  50  kw.  of  exciter  output, 

15 


226 


CENTRAL  STATIONS 


[ART.  274 


which  is  equivalent  to,  approximately,  a  2,000-kw.  alternating- 
current-generator  output.     Where  the  capacity  of  the  exciter 


FIG    192. — Connections  of  small-capacity  regulators  with  one  arrangement 
of  two  exciters  in  parallel. 


,'A.C.Control  Magnet 

Compensating  Winding 
.-Not  Used 

Otenria! 
.-Winding 


ftfp* 

o      y 

°o      Reactance 

r  °o 
°9     °0-°C 

?°o 

0 
0 

0 
0 
0 

°°D 

1  , 

1 

L 


X/ 


3-ffiase  tine-'---  Current  Transformers'-''1 

Fia.  193. — Arrangement  of  a  line-drop  compensator  for  a  voltage  regulator. 

is  in  excess  of  about  50  kw.,  additional  contacts  must  be  ar- 
ranged in  series,  each  shunting  a  portion  of  the  exciter-field 
rheostat. 

274.  The  Operation  of  Generator  Voltage  Regulators  in 


SEC.  12] 


AUTOMATIC  VOLTAGE  REGULATORS 


227 


Parallel  is  the  practice  in  many  important  installations  in 
which  each  generator  has  an  individual  exciter  and  regulator, 
the  combination  comprising  a  complete  and  distinct  unit. 
Fig.  192  illustrates  an  arrangement  of  this  type.  Cross-cur- 
rents between  the  generators,  which  might  occur  because  of 
the  exciters  having  different  characteristics,  are  eliminated  by 
a  certain  arrangement  of  voltage  and  current  transformers. 


.Current  Transformer 


FIG.  194. — Arrangement  of  an  automatic  regulator  for  exciters  of  small 
capacities  controlling  several  alternating-current  generators  in  parallel  with 
their  exciters  in  parallel. 


275.  Compensation  for  Line  Drop  is  effected  by  means  of  a 
compensating  winding  (K,  Fig.  188).  The  object  of  this  com- 
pensation is  to  maintain,  as  nearly  as  practicable,  a  constant 
voltage  at  a  center  of  distribution  out  on  the  line  distant  from 
the  generator  and  station.  As  the  current  through  LI  (Fig. 
188)  increases  the  excitation  of  K  increases  accordingly.  The 
pull  of  K  being  proportional  to  the  line  current  is,  in  general, 
proportional  to  the  line  drop.  Therefore,  the  pull  of  K,  in 
combination  with  that  of  A,  can  be  so  proportioned  that  the 
average  line  drop  will  be  compensated  for  by  the  regulator. 


228 


CENTRAL  STATIONS 


[ART.  276 


A  dial  switch  is  provided  in  combination  with  K  to  provide 
the  proper  value  of  compensation  for  the  feeder  circuit  in 
which  current  transformer  J  is  inserted.  A  special  compensa- 
tor (Fig.  193)  is  provided  where  it  is  necessary  to  compensate 
for  both  resistive  and  inductive  drop. 

276.  Connections  for  Voltage  Regulators  for  Different 
Services  are  shown  in  Figs.  192  and  194.  There  are  almost 
innumerable  possibilities  in  the  arrangement  of  these  regu- 
lators for  different  services.  Those  illustrated  are  typical. 


FIG.  195. — Regulator  mounted  at  side  of  a  switchboard  panel. 

277.  In  Installing  Voltage  Regulators  they  may  be  arranged 
at  the  end  of  a  switchboard  attached  to  one  of  the  panels  as 
shown  in  Fig.  195  or  they  can,  if  mounted  on  a  standard- 
panel  section  (Fig.  189),  be  incorporated  directly  in  a  switch- 
board. It  is  also  feasible  to  mount  a  unit  like  that  of  Fig.  190 
on  the  front  of  a  switchboard. 


SECTION  13 
SWITCHBOARDS  AND  SWITCHGEAR 

278.  The  Distinction  Between  "Switchboard"  and  "Switch- 
gear"  should  be  understood.  By  definition  "switchgear  con- 
stitutes the  parts  or  appliances,  collectively,  which  make  up 
a  complete  equipment  for  controlling  and  metering  the  elec- 


.  -.Ground  Detector  Lamps       ..-Generator  Panels-. 


Fia.  196. — Small  "standard-unit"  switchboard  for  two  compound- 
wound  direct-current  generators  and  seven  feeders.  (General  Electric 
Company.) 

trical  energy  output  or  input  of  an  electrical  station  or  some 
electrical  device." 

279.  The  Function  of  a  Switchboard  may  be  explained  thus: 
A  switchboard  is  that  component  of  a  switchgear  equipment 
on  which  are  mounted  the  meters,  switch-control  handles, 
229 


230 


CENTRAL  STATIONS 


[ART.  279 


rheostat  handles  and  similar  contrivances.  For  the  control 
of  small  amounts  of  power  at  low  voltages  it  is,  as  will  be  shown, 
most  convenient  and  economical  to  mount  all  of  the  switch- 
gear  on  the  switchboard,  in  which  case  the  board  is  then  said 
to  be  self-contained.  Where  the  power  output  is  large  or  at 

Circuit          Panels  are  Natural  Black  Slate,  t\  In.  Thick  with  j  In.  Bevel  Except 
r  Breakers        Those  above  3, 000  Amp.  Panel  Rating,  Which  are  2  In,  Thick 


Hand  (6g\Wheel 
&Pot.  Rec 


Main 
Switch 


!r 


I.     c 


Synchronous  Converter  Panels 


Feeder  Pane! 


Note:-  Paneis  over  4000  Amp.  have  Top  Sections  40  In.  High 
ana"  for  Fig.  A    anal  C  are  70  In,  Wide. 
Oenerator  and  Converter  Panels  4,000  Amp.  and  Less 
for  Installation  in  the  Same  Board  with  Larger  Panels 
will  also  be  Furnished  with  40  In.  Top  Sections. 

Fia.  197. — Unit   switchboard   panels   for   600-volt,    direct-current,    railway 
service.  * 

high  voltage,  it  is  necessary  to  install  certain  components  of 
the  switchgear  at  locations  distant  from  the  switchboard 
proper.  That  is,  under  those  conditions  "remote  control"  is 
utilized.  A  switchboard  which  involves  remote  control  is 
called  a  remote-control  switchboard.  However,  in  general,  the 
control  is  always  effected  from  and  by  means  of  switches, 
meters  and  appliances  on  the  switchboard. 

*  General  Electric  Company. 


SBC.  131 


SWITCHBOARDS  AND  SWITCHOEAR 


231 


280.  Switchboards  May  Be  Divided  Into  Four  Classes: 

Panel  switchboards  (Figs.  196  and  197) ;  control-desk  switch- 
boards (Fig.  198) ;  pedestal  switchboards  (Figs.  199  and  200) ; 
and  post  switchboards  (Fig.  201). 


Imrrumenr--' 


!* 2ff.  Sin •: 

FIG.  198. — End  elevation  of  the  con- 
trol desk  and  instrument  board. 


Fio.  199. — Westinghouse 
equalizer  pedestal  with  switch 
for  compound-wound  direct- 
current  generator. 


281.  A  Panel  Switchboard  (Figs.  196  and  197)  is  one  com- 
posed of  panels  of  insulating  material  supported  on  a  suitable 
iron  framework.     The  various  switches,  instruments,  rheostat 
handles  and  other  control  appliances  are  mounted  on  these 
vertical  panels.     Each  panel  is,  for  the  larger  switchboards, 
composed  of  sections.     The  panels  are  mounted  side  by  side 
to  constitute  a  complete  switchboard. 

282.  The  Procedure  in  Laying  Out  a  Switchboard  is  indi- 
cated in  an  elementary  way  in  Figs.  202,  203,  204  and  205. 


232 


CENTRAL  STATIONS 


[ART.  282 


'Lamp  Indicators 


FIG.  200. — Westinghouse  control 
pedestal  used  in  combination  with 
an  instrument  post. 


FIG.  201. — Instrument  post 
used  instead  of  a  panel 
switchboard. 


'-D.C.denerator 
FIQ.  202 — Single-line  diagram  for  the  direct-current  switchboard. 


SEC.  13] 


SWITCHBOARD  AND  SWITCHGEAR 


233 


These  show  the  control  and  measuring  equipment  for  a  small 
direct-current  compound-wound  generator  which  serves  four 
feeder  circuits.  The  first  step  is  to  make  a  simple  "single- 
line"  diagram  (Fig.  202)  indicating  what  it  is  desired  to  ac- 
complish with  the  switchboard.  Such  a  diagram  usually  shows 
only  the  important  elements  such  as  switches,  fuses,  circuit- 
breakers  and  the  like  and  their  general  relation  to  one  another. 
Then  as  a  second  step  a  circuit  diagram  (Fig.  203),  which  shows 
all  of  the  devices  which  are  to  be  incorporated,  can  be  made. 


•Series  Field  Winding 


"""***«»  X 

Ingle  -Pole 
ircuit  Breaker     \ 

i-Wfmeter    ^.j 
Switch, 

+ 

Ammeter     .•' 
,'Shunt 

"kt^Z—. 

™   H     G 

W  *~4  ; 
^         ffJ 

Switches'      f 

i 

i 

V 

, 

f 

IF.!     !F..|     |F-|     Ir.l 

FIG.  203. — Circuit  diagram  for  switchboard  lay-out  for  a  single  compound- 
wound  generator. 

Working  from  this  basis  the  equipment  can  be  arranged  on  the 
front  of  an  insulating  panel  of  suitable  size  (Fig.  204).  Then 
the  arrangement  of  the  apparatus  and  wiring  on  the  back  of  the 
board  can  be  worked  out  as  suggested  in  Fig.  205.  After  the 
layout  of  the  back  of  the  board  has  been  examined  it  may  be 
necessary  to  alter  the  locations  of  certain  equipment  on  the 
front.  The  front  and  rear  layouts  must  be  developed  in 
conjunction.  It  is  not  practicable  to  develop  one  inde- 
pendently of  the  other.  The  lettering  on  Fig.  203  corresponds 
with  that  on  Fig.  205. 

283.  In  Arranging  the  Switchboard  Panels  it  is  usually 
desirable  to  locate  the  generator  panels  (Fig.  206)  at  the  left 
and  the  feeder  panels  at  the  right,  although  it  may  be  advisable 
to  depart  from  this  practice  under  certain  conditions.  Totaliz- 


234 


CENTRAL  STATIONS 


[ART.  284 


ing  panels  or  tie-bus  panels  may  be  inserted  between  the 
generator  and  feeder  panels  as  shown  at  T,  Fig.  206. 


FIG.  204. — Front  view  of  switch- 
board panel  for  a  single  compound- 
wound  direct-current  generator. 


FIG.  205. — Phantom  view  of  the 
switchboard  panel  for  the  compound- 
wound  generator  as  it  would  appear 
if  the  panel  were  removed  exposing 
the  connections  behind. 


•     .•  -Bus  Tie  Panels  Usually  Between  Generators 
Ty    and  Feeders.  When  Included     


-Generator  Panels— • **i* — Feeder  Panels 


Right 
FIG.  206. — Showing   usual   arrangement   of   switchboard   panels  * 

284.  The  Proportions  of  Switchboard  Panels  and  Sections 

have,  in  the  United  States,  been  fairly  well  standardized  for 

*  General  Electric  Company. 


SBC.  13] 


SWITCHBOARD  AND  SWITCHGEAR 


235 


self-contained  switchboards.  Panels  for  switchboards  of 
medium  or  large  capacity  are  almost  universally  made  90  in. 
high  (Fig.  197).  The  "unit"  sections  of  a  90-in.-high  switch- 
board are,  in  accordance  with  the  practice  of  one  manufac- 
turer, of  the  heights  indicated  in  Fig.  197.  Where  only  two 
sections  constitute  a  panel,  the  lower  slab  may  be  25  in.  high 
and  the  upper  slab  65  in.  A  different  manufacturer  uses  a 
62-in.-high  upper  slab  and  a  28-in.-high  lower  slab.  The 
general  practice  is  now,  however,  to  always  use  three  sections 
for  90-in.-high  panels,  in  which  case  the  section  heights  may  be 
as  shown  in  Fig.  197,  or  instead,  the  upper  section  may  be  20 
in.  high,  the  middle  section  45  in.  and  the  lower  25  in.  high. 

NOTE. — It  has  been  explained  that  the  reason  why  these  particular 
dimensions  were  adopted  is  that  a  20-in.-high  section  at  the  top  is  of 
ample  proportions  to  support  the  standard  brush-type  carbon  circuit- 
breaker,  which  is  often  located  at  the  top  on  the  switchboard  so  that  the 
arc  which  rises  from  it,  when  it  operates  under  load,  cannot  do  damage. 
The  heights  of  small  single-section  panel  switchboards  like  that  of  Fig. 
196  have  not  been  thoroughly  standardized.  One  company  uses  a 
height  overall  of  5  ft.  4  in.,  where  feasible,  for  boards  of  this  general  design. 

i'.    i'x  if  Strap  mn> 


' 

'      i            1 

Cast 

Iron 

Panel 

Support, 

' 

•<          *-"    t 

*.  i 

«•/!* 

u 

**'**         1 

?                1 

Cast 

lj*GasPipe 

Floor 

-Threaded 

r- 

"*-.     71 

Panet-..^ 

-Mounting 
Brackets 

\ 

'.             -j 

Pane/Brace. 
™         % 

}'  '  Wrought-.J:    ' 

,/<** 

%*-->. 

mn°r    / 

.-Flangt 

'!ji  1 

FIG.  207. — Wrought-iron  pipe  frame 
for  switchboard.  * 


I- Rear  View  I- Side  View 

FIG.    208. — Wrought-iron  pipe  sup- 
port for  small  panel. 


285.  The  Frames  for  Panel  Switchboards  are  made  either 
of  wrought-iron  pipe  or  structural-steel  sections.     The  com- 

*  Westinghouse  Elec.  &  Mfg.  Co. 


236 


CENTRAL  STATIONS 


[ART.  286 


ponents  of  the  pipe  frames  are  shown  in  Figs.  207  and  208  for 
90-in.  and  64-in.-high  boards  respectively.  The  pipe  frames 
appear  to  be  becoming  more  popular.  The  general  construc- 
tion is  apparent  from  the  illustration.  Standard  angle-iron 
frames  are  constructed  as  illustrated  in  Fig.  209.  The  angle- 


/If 


9.%?< 

•V  j   J 

Corner  / 

-•i 

2'*3"xj* 

**       * 

-•*• 

.  .  Jr                      o 

2"xjV 

Angle  Iron-  -' 

:J  <• 

i 

,2" 

,-•/# 

'-,i*Hote 

t£^LJ 

1====* 

6  *-<9ife  -per-Foo-t-Channel  Base  - 
FIG.  209. — Angle-iron  frame  for  switchboard.  * 

iron  frame  provides  a  very  rigid  support  lor  the  switchboard 
panel.  In  every  installation  a  substantial  channel  base,  C 
(Fig.  209),  should  be  provided  to  form  a  level  and  substantial 
footing  on  which  the  upright  pipe  or  single  members  which 
support  the  panel  can  rest. 

286.  Fittings  for  Supporting  Switchgear,  sometimes  called 
"switchgear  details,"  can  now  be  obtained  in  many  different 
designs,  some  one  of  which  will  satisfy  almost  any  condition. 
Figs.  210  and  211  show  typical  forms.  The  drawing  of  Fig. 
211  details  the  fittings  for  an  iron  pipe  brace  which  may  be 

*  Westinghouse  Electric  &  Mfg.  Co. 


SEC.  13] 


SWITCHBOARD  AND  SWITCHGEAR 


237 


used  for  bracing  a  panel  to  the  floor,  as  shown  in  Fig.  208,  or 
for  attaching  it  to  a  wall. 

287.  The  Material  for  Switchboard  Panel  Sections  must  be 


\$ 

1 

FIG.  210. 


Fittings  and  appliances  which  are  regularly  manufactured  for 
switching  installations.  * 


an  insulator.  Either  slate  or  marble  is  now  ordinarily  used 
for  this  service.  Slate  costs  less  than  marble  and  is  stronger 
but  is  not  as  good  an  insulator;  therefore,  where  the  pressure 

*  Electric  Journal,  May,  1913,  p.  82. 


238 


CENTRAL  STATIONS 


[ART.  288 


for  which  insulation  must  be  provided  is  more  than  750  volts 
or  less  than  1,100  volts  the  use  of  marble  is  imperative.  How- 
ever, modern  switchboards  (even  those  for  the  control  of  appa- 
ratus operating  at  the  highest  commercial  voltages)  do  not 
have  extending  through  them,  conducting  members  the  differ- 
ence of  potential  between  which  exceeds  110  volts.  Even 
for  2,400  and  6,600-volt  switchboards  the  oil  switches,  the 
instrument  transformers  and  the  other  members  upon  which 
line  voltage  is  impressed  are  thoroughly  insulated  from  the 
switchboard  sections. 


Threaetnt-Type-Brace-,: 
Pipe  Clamp 


FIG.  211. — Wall  or  floor  brace. 

NOTE. — Therefore,  slate  sections  can,  from  an  electrical  standpoint, 
be  used  for  practically  any  switchboard.  The  marble  sections  soil 
easily  and  are,  under  certain  conditions,  almost  impossible  to  clean.  The 
black-finished  slate  panels  will  always  look  well  if  reasonable  attention  is 
given  to  them.  There  does  not  appear  to  be  any  justification  for  the  use 
of  marble  as  a  switchboard  section  material  except  in  display  installations. 

288.  Panel  Switchboards  Are  Used  more  frequently  than 
those  of  any  of  the  other  types  because  of  their  adaptability 
to  the  many  different  conditions.  They  may  be  utilized  for 
the  control  of  practically  any  direct  or  alternating-current  in- 
stallation. However,  as  will  be  explained,  switchboards  of  the 
pedestal,  post  and  control-desk  types  may  be  desirable  or 
necessary  for  large  or  complicated  installations.  Self-con- 
tained panel  switchboards  may  be  used  where  the  voltage  does 


SEC.  13]  SWITCHBOARD  AND  SWITCHGEAR  239 

not  exceed  6,600.     Remote-control  panel  switchboards  can 
be  used  for  equipment  operating  at  higher  voltages. 

289.  Post  Switchboards  (Fig.  201)  are  practically  always  of 
the  remote-control  type.     They  may  be  used  in  large  installa- 
tions wherein  they  may  be  so  located  that  they  can  be  readily 
observed  by  the  operator  without  obstructing  his  general  view 
of  the  station  interior. 

290.  Pedestal  Switchboards  (Figs.  199  and  200)  are  some- 
times called  control  pedestals.     These  of  the  type  suggested 
in  Fig.  200  are  used  in  conjunction  with  instrument  posts  for 
controlling  generator  or  feeder  circuits.     One  pedestal  may  be 
provided  for  each  generating  unit,  which  minimizes  the  possi- 
bility of  the  switchboard  operator  effecting  misconnections. 
The  pedestals  are  sufficiently  low  that  they  do  not  interfere 
with  the  operator  observing  the  interior  of  the  entire  station. 
Equalizer  pedestals  (Fig.  199)  are  used  for  supporting  the 
equalizer  switches  for  compound-wound  direct-current  gen- 
erators.    Such  a  pedestal  may  be  located  near  each  large 
direct-current  generator  so  that  the  cost  of  the  relatively  large 
cables  to  the  switchboard  proper,  which  would  otherwise  be 
necessary,   is  eliminated.     The  application  of  an  equalizer 
pedestal  is  shown  in  a  following  illustration  under  the  heading 
"600-volt  Railway  Switchboards." 

291.  Control-desk  Switchboards  (Fig.  198)  are  always  of  the 
remote-control  type  and  are  ordinarily  desirable  only  for  large 
installations.    The  instruments  are  usually  arranged  on  vertical 
panels.     In  the  face  of  the  control  desk  are  arranged  the  re- 
mote-control switches  and  indicating  lamps  and  on  its  face  is 
frequently  mounted  a  miniature  bus  structure  whereby  the 
operator  can  observe  at  any  time  the  conbination  of  inter- 
connections then  existing  between  the  generating  and  convert- 
ing equipment  in  the  station  and  the  lines  or  feeders  radiating 
from  it. 

292.  Direct-current  Switchboards  are  practically  always  of 
the  panel  self-contained  type,  with  the  exception  that  equal- 
izer pedestals  (Fig.  199)  may  be,  in  large  installations,  used  in 
combination  with  them.     (The  statements  just  preceding  ap- 
ply to  direct-current  switchboards  for  pressures  not  exceed- 


240 


CENTRAL  STATIONS 


FART.  293 


ing  600  volts.  Recently  direct  pressures  of  1,200,  1,500,  2,400 
and  3,000  volts  have  been  proposed  for  long-distance  electric 
railways.  With  these  relatively  high  voltages  remote-con- 
trol switchboards  of  either  the  mechanical  or  electrical  types 
may,  under  certain  conditions,  be  desirable. 

293.  Direct-current  Switchboards  for  Small-capacity  In- 
stallations may  be  of  the  general  single-section  panel  design 
suggested  in  Fig.  196,  which  shows  a  switchboard  for  the  con- 
trol of  two  compound-wound  generators  and  seven  feeders. 


•Neutral  Main  Bus 


Oenei 

I  Wire  _ 


2Wire---x J  Wire--- 


-  .......  Feeder  Panels 

-3Wi're~*.2 


frntnoW 

Neyativf 


FIG.  212. — Typical  low-voltage,  direct-current,  switch-board  connections. 

Gi  and  G2  are  the  generator  switches.  The  middle  blade  of 
each  of  these  switches  is  in  the  equalizer  lead  and  is  unfused. 
The  feeders  Fit  Fz  and  Fs,  for  the  motors,  are  protected  by 
circuit-breakers.  The  lighting  feeders,  Ft)  F5  ,F6  and  F^  are 
protected  by  enclosed  fuses. 

294.  Direct-current    Switchboards    for     Installations     of 
Medium  and  Large  Capacity,  are  practically  always  of  the 
three-section  panel  type  (Fig.  197)  and  are  90  in.  high. 

295.  The  Essential  Circuit  Diagrams  for  Direct-current 


SEC.  13] 


SWITCHBOARD  AND  SW1TCHGEAR 


241 


Switchboards  are  shown  in  Fig.  212.  This  indicates  how  two- 
wire  and  three-wire  generators  are  connected  and  how  one- 
wire,  two-wire  and  three-wire  feeders  may  be  arranged. 

296.  A  Moderate  -  capacity,  Two  -  wire,  Direct  -  current 
Switchboard  is  shown  in  Figs.  213,  214  and  215.  These 
illustrations*  show  the  general  construction  of  a  unit  section, 


.-Circuit  Breakers 


Double  Pole  Circuit  Snitches 
Provided  with  Enclosed  Fuses 


'-Panel  for  Generator  N«l      ''Panel  for  General  or  N?  2 

FIG.  213. — Front  view  of  the  five-panel  direct-cuijrent  switchboard. 

direct-current  switchboard  for  a  medium-capacity,  isolated- 
plant  installation  operating  at  a  pressure  of  110  volts.  Fig. 
216  shows  the  connection  diagram.  Each  lighting  feeder  is 
controlled  by  a  double-pole  knife  switch  protected  with  en- 
closed fuses.  Each  motor  feeder  is  controlled  and  protected 
by  a  double-pole  automatic  circuit-breaker.  Each  of  the  two 

*  Roland,  APPLIED  ELECTRICITY  FOE  PBACTICAL  MEN,  p.  100. 
16 


242 


CENTRAL  STATIONS 


[ART.  297 


generators  has  its  own  panel  and  is  protected  by  a  double- 
pole  circuit-breaker,  Bi  and  #2.  In  a  moderate  capacity 
installation  such  as  this  the  equalizer  lead  from  each  generator 
is  carried  to  the  switchboard  and  connected  to  the  center 
blade,  EI  and  E2  (Fig.  213)  of  the  main  switch. 


Main  Fuses, 
Enclosed 


Section  at  a  Generator  Panel 

FIG.  214. — Vertical  section  taken  just  to  the  right  of  one  of  the  generator 
panels. 

297.  Large -capacity  Two-wire  Direct-current  Switch- 
boards are  usually  arranged  so  that  an  isolated  equalizer 
pedestal  (Fig.  199)  may  be  used  for  economic  reasons  above 
outlined.  Fig.  217  diagrams  typical  connections  for  a 
generator  panel  of  this  type.  The  generator  panel  itself  is 
illustrated  in  Fig.  218.  The  feeder  panels  are  usually 
constructed  about  as  shown  in  Fig.  219. 


SEC.  13] 


SWITCHBOARD  AND  SWITCHGEAR 


243 


298.  Three-wire  Direct-current  Switchboards  resemble,  in 
general  external  appearance,  those  for  two-wire  circuits.  The 
connections,  however,  are  materially  different  in  certain  de- 
tails, as  disclosed  by  Fig.  220.  This  delineates  the  circuit 
arrangement  for  a  switchboard  serving  two  three-wire  110- 


.....  ...      ••;    />  -  -       •  •    •          ••  -  - 

FIG.  215. — Rear    elevation    of    the    five-panel    direct-current    switchboard. 

220-volt  direct-current  generators  and  three  feeders,  one  of 
the  feeders  being  220-volt  two-wire. 

299.  Switchboards  for  600-volt  Direct-current  Railway 
Service  are  really  two-wire  switchboards  but  due  to  the  fact 
that  a  ground  return  is  almost  always  used  for  railway  cir- 
cuits, certain  variations  from  the  standard  two-wire  con- 
struction is  necessary.  Only  one  side  of  the  circuit,  usually 
the  positive,  is  carried  to  the  switchboard  as  illustrated  in 


244 


CENTRAL  STATIONS 


[ART.  300 


Fig.  221.  Both  the  equalizer  switch  and  the  negative  switch, 
if  such  is  used,  may  be  mounted  on  a  pedestal,  PI  and  P2  (see 
Fig.  199  for  detail)  located  near  the  machine.  The  negative 
side  of  the  line  is  carried  to  a  ground  connection  near  the 
pedestal.  Hence,  in  effect,  the  earth  itself  constitutes  the 
negative  bus.  By  utilizing  this  "single-bus"  design  material 


FIG.  216. — Wiring  diagram  for    the    five-panel  direct-current   switchboard. 

economies  in  first  cost  are  realized.  Both  the  generator 
and  feeder  panels  (Fig.  197)  can  then  be  made  narrower 
because  only  a  single-pole  switch  is  necessary  on  the  panels. 
This  tends  to  reduce  the  cost  of  the  switchboard.  A  further 
economy  results  from  the  fact  that  with  the  single-bus  arrange- 
ment it  is  not  necessary  to  route  the  heavy  negative  and 
equalizer  conductors  to  the  switchboard. 

300.  Alternating-current  Switchboards  may  be  divided  into 
two  general  classes:  (1)  self-contained,  and  (2)  remote-control. 


SEC.  13] 


SWITCHBOARD  AND  SWITCHGEAR 


245 


The  remote-control  boards  can  be  further  subdivided  into: 
(a)  mechanical  remote-control,  and  (6)  electrical  remote-control. 
It  is  usually  good  practice  for  all  except  extraordinary  con- 
ditions, to  use  self-contained  switchboards  for  alternating- 
current  service  at  pressures  not  exceeding  6,600  volts  where 
the  capacity  of  the  station  does  not*  exceed  about  3,000  kva. 
Where  the  capacity  or  the  voltage  exceeds  the  value  just  noted, 


For  600  Volt- 
Circuits  Omit 
thtt 

Connect  Lower 
Studs  of  Ammeter ' 
Together 


Equalizer  Bus-* 

FIG.  217. — Low-voltage  direct-current  generator  panel  wiring  where  equalizer 
pedestal  is  used.     (Back  view.) 


remote-control  equipments  should  be  used.  A  feature  which 
distinguishes  alternating  from  direct-current  switchboards  is 
that  it  is  standard  practice  to  use  oil  switches  instead  of  air- 
break  switches  for  rupturing  the  alternating-current  circuits 
on  voltages  as  low  as  even  240.  Inasmuch  as  the  three-phase 
system  is  now  adopted  for  almost  every  energy-generating 
and  transmitting  installation,  a  majority  of  the  alternating- 

•C.  H.  Sanderson,  "SWITCHBOARDS  FOR  ALTEBNATINO-CUBBENT  POWEB  STATIONS." 


246 


CENTRAL  STATIONS 


[ART.  300 


•Circuit 
Breaker 


Hanoi 
Wheel 


k 24  In. 

I- Front  View 


Watthour 
Meter 


FIG.  218. — Low-voltage,  direct-current,   two-wire  generator  pane)  of  large 
capacity. 


.-Circuit 

Breakers 


•Feeder 
Switches 


Terminal- 
Lugs 


i 


I-Front-Elevcrtion 


E-Rear  Elevation 


~m-Side  Elevation 


FIQ.  219. — Typical  direct-current  feeder  panel  in  an  installation  of  con- 
siderable capacity. 


SEC.  13] 


SWITCHBOARD  AND  SWITCHGEAR 


247 


3  Wire 

Feeder         .    . 

with  C.B      3  Wire 

Fused 

Feeder 


FIG.  220. — Wiring  diagram   for   a   three- wire,    direct-current   switchboard 
serving  two  generators  and  three  feeders. 


FIG.  221.— Typical  connection  diagram  for  a  600-volt,  direct^current  railway 
switchboard. 


248 


CENTRAL  STATIONS 


[ART.  301 


current  switchboards  which  are  now  installed  are  for  three- 
phase  service. 

301.  Alternating-current  Switchboards  for  Three-phase 
240-  and  480-volt  Service  are  (except  those  of  great  capacity) 
nearly  always  self-contained.  The  wiring  diagram  for  a 
typical  outfit  of  this  character  is  shown  in  Fig.  222.  The 
general  appearance  of  such  a  board  would  be  the  same  as  that 
for  a  2,400-volt  board  shown  in  Fig.  223.  It  should  be  noted 


'  Connections  are  Shorm  m  Viewed  from  Rear  of  Board. 
9 =250  Y.I  Amp.  Fuse;  X  -600V.IAmp.ruse;  8= Lamps  on Rear. 
Connect  Auto  Starter  as  per  Diagram  Furnished  with  Slxrter,. 

FIG.  222. — Typical  wiring  diagram  for  a  240-  or  a  480-volt  switchboard. ' 


that  oil  switches  are  used  for  the  three-phase  switches  in  this 
installation. 

301A.  Switchboards  for  2,200  to  2,400-volt  Three-phase 
Alternating-current  Service  f  are  also  nearly  always  self-con- 
tained. Figs.  223,  224  and  225  show  the  external  appearance 
of  a  typical  switchboard  of  this  character  while  Fig.  226 
delineates  the  detailed  wiring  diagram.  Fig.  227  shows  a 
single-line  diagram  of  the  board.  This  equipment  is  typical 
of  that  which  would  be  used  in  a  central  station  which  sup- 
plies light  and  power  to  a  small  city.  Two  generators,  G\  and 
Gt  (Fig.  227),  and  two  exciters,  E\  and  E%,  serve,  together, 

*  Westinghouse  Electric  &  Manufacturing  Co. 

t  C.  H.  Sanderson,  "SWITCHBOARDS  FOB  ALTERNATING-CURRENT  POWER  STATIONS." 


SEC.  13] 


SWITCHBOARD  AND  SWITCHGEAR 


249 


four  feeders,  F i  to  F4.  Panels  1  and  2  are  the  combination 
generator-and-exciter  panels.  An  automatic  voltage  regu- 
lator is  mounted  at  the  end  of  the  board  on  panel  No.  1. 
Panel  3  serves  two  three-phase  power  feeders.  Panel  4  con- 


Fio.  223. — Perspective  view  of  the  typical  2400- volt,  three-phase  switchboard 
for  power  and  lighting  service  in  a  small  town. 

trols  the  rectifier  circuits  for  the  series  direct-current  arc 
lighting.  Panel  5  carries  the  auto-starter  and  ammeter  for 
the  alternating-current  motor  end  of  a  synchronous  motor- 
generator  set  which  supplies  the  town  with  direct  current. 
Less  expensive  switchboards  of  the  general  construction  indi- 


250 


CENTRAL  STATIONS 


FART.  301 


bobotolo' 


ftft 


.  Sw.- 
2 


•Lamp* 

Oil  5» 

f 
V 


Auto- 
Starter 


Fio.  224. — Front  view  of  the  2400- volt,  three-phase  lighting  and  power 
switchboard. 


,-Bus  Bars 


'Volt  Transformers 


Fio.  225. — Rear  elevation  of  the  switchboard  the  perspective  and  front 
views  of  which  are  shown  in  other  illustrations. 


SEC.  13]  SWITCHBOARD  AND  SW1TCHGEAR 


251 


252 


CENTRAL  STATIONS 


[ART.  301 


cated  in  Fig.  196  may  be  purchased  for  simple  alternating- 
current  installations  but  inasmuch  as  plenty  of  room  is  desir- 
able on  an  alternating-current  switchboard  the  90-in.-high 
type  (Fig.  223)  is  usually  preferable. 


NOTE. — The  advantages  accruing  through  the  use  of  remote-mechan- 
ical-control switchboards  as  compared  with  self-contained  switchboards 
have  been  thus  summarized  by  C.  H.  Sanderson  in  his  SWITCHBOARDS 
FOR  ALTERNATING-CURRENT  POWER  STATIONS:  (1)  All  high  voltages 
are  removed  from  the  panels,  thus  permitting  ready  inspection  of  the 
instrument  and  control  wiring,  eliminating  danger  of  injury  to  attendants 
from  contact  with  live  parts,  permitting  the  location  of  the  board  to 
much  better  advantage  as  regards 
the  remainder  of  the  installation  be- 
cause less  space  and  less  protection 
are  required.  (2)  Panels  are  not 
subject  to  the  mechanical  strains 
due  to  automatic  operation  or  to  the 
dead  weight  of  the  apparatus.  (3) 
In  case  of  marble  panels  their  ap- 
pearance  is  not  marred  by  stains 

FiG.^T.-Single-line  diagram  of  the  from  *"&**  °IL     .<4)  Violent  explo- 
switchboard  of  Fig.  226.  Slons   which    sometimes    occur    upon 

the  opening  of  heavy  currents  or  the 

possible  failure  of  a  circuit-breaker  will  not  injure  the  panels  and,  if  the 
circuit-breakers  are  sufficiently  spaced  or  are  enclosed  in  fireproof  cells, 
adjacent  circuit-breakers  will  not  be  affected.  (5)  The  panels  may  be 
much  narrower,  the  reduced  cost  thereof  off-setting,  to  a  considerable 
extent,  the  additional  cost  of  the  remote-control  feature.  Moreover, 
the  decrease  in  total  length  of  the  board  may  result  in  a  very  material 
saving  in  cost.  (6)  A  more  compact  arrangement  of  the  apparatus  is 
of  great  assistance  to  the  operator,  approaching  as  it  does  more  nearly 
to  the  compact  and  efficient  arrangements  obtained  by  means  of  control 
desks.  (7)  Much  shorter  main  connections  are  made  possible,  and  high 
voltages  kept  away  from  certain  floors,  or  certain  rooms  by  locating  the 
remote-control  structure  properly.  (8)  Where  a  wall  is  used  for  sup- 
porting the  apparatus,  the  cost  of  the  complete  outfit  may  be  reduced 
to  very  near  that  of  the  self-contained  type  of  board,  and,  in  some  cases 
of  very  heavy  capacities  at  low  voltages,  may  be  less  in  cost.  Moreover, 
accessible  arrangements  of  apparatus  with  ample  spacings  may  easily 
be  obtained.  (9)  Where  a  steel  or  masonry  structure  is  used,  access 
may  be  had  to  either  side  of  the  structure  and  an  arrangement  of  this 
kind  will  satisfactorily  accommodate  the  maximum  amount  of  apparatus 
ordinarily  used  for  either  single-  or  double-throw  arrangements. 


SEC.  131 


SWITCHBOARD  AND  SWITCHGEAR 


253 


302.  An  Alternating-current  Mechanical  Remote-control 
Switchboard  is  shown  in  Figs.  228,  229,  and  230.  A  wiring 
diagram  is  given  in  Fig.  231.  Boards  of  this  general  design 

InettYm.-, 
Voltmeters.        mmefers:      •A.C.Am-*.-lnd.Wm.i  ,A.C.Am..-'    .-Ammeters, 


D 


Oil 

Circuit 
Breaker 


K--  2  Ft.  ••••»<<  -ingin.  »t/Ft  8  In.*  I  Ft  8  In. 
k~  .....  —  ..........................  J0ft4ln- 


FIQ.  228.  —  Front  view  of  the  mechanically-operated  remote-control  switch- 
board. 

are  suitable  for  applications  for  voltages  not  exceeding  35,000 
and  capacities  not  exceeding  25,000  kva.  three-phase.*  It 
should  be  noted  that  all  of  the  alternating-current  control 


flail-' 
FIG.  229. — Plan  view  of  the  mechanical  remote-control  switchboard. 

apparatus  is  supported  on  a  structure  independent  of  the 
switchboard  panels.  The  control  equipment  can,  for  relatively 
low  voltages,  be  supported  on  a  wall  directly  back  of  the 

*  C.  H.  Sanderson,  SWITCHBOARDS  FOB  ALTERNATINO-CUBRENT  POWER  STATIONS. 


254 


CENTRAL  STATIONS 


[ART.  302 


fxc/ter      Am.      Instrument 
'Bus  Bars   Shunts.      Busses. 


It -Rear  Elevation 


Remote  Mechanical-'         ''Current  Trans  former 
Control  Operating  Rod 

I-Sectional  Elevation 

Fia.  230. — Rear  view  and  sectional  elevation  of  remote-control  switchboard 
mechanically  operated. 


:  are  Shown  as  Viewed  From 
Rear  of  Board.    0  -  250  V.  I  Amp.  Fuse. 
_jj  =  250  V.  2  Amp.  FUS& 

FIG.  231. — Wiring  diagram  for  a  typical  mechanical  remote-control  switch- 
board. 


SEC.  13 


SWITCHBOARD  AND  SWITCHGEAR 


255 


Tf"'s  Distance  Varies 
{to  Suit  Apparatus 


Pipe-'    ftcite 
Brace    Bus 
Bars 

^-Enclosed 
-?•  Cell  if 
:'.'•  Desired      Wire 
:'.',  Asbestos    Rope  to- 
Lumber    Rheostat 

•Oil  Circuit 
Breaker 


FIG.  232. — Mechanical  remote- 
control  circuit-breaker  mounted  on 
wall. 


FIG.  233. — Arrangement  of  me- 
chanical remote-control  switchgear 
utilizing  wall  and  a  pipe  frame  for  a 
supporting  structure.* 


BeJICrank- 

Remote  Control  Connecting  Rotl- 
FIG.  234.— Remote    mechanical    oil    circuit-breaker   arranged    m    masonry 
structure.  * 


C.  H,  Sanderson  in  "SWITCHBOARDS  FOR  A.  C.  POWER  STATIONS." 


256 


CENTRAL  STATIONS 


[ART  302 


u       Generator         '    Line 

' Sections     ",Sectio 

Voltage  Line  Circuit 


;5ectiona!izina  Section 
*''-<      Uxneratvr      Line 
'    '^Section  ^ Section 


Generator 
Sections 


I  -  Plan  of  Slopiog  Top  of  Control  Desk 


Generator  Vnltmeter 

Ammeter  Indicating  .Voltmeter 

Transformer          Wattmeter   power  Factor        ',  field  Ammeter 
;        ,'Ammeter      ..Ammeters      !  .-Meter  !'. 

•       ;  '\  ;   \  ;5ynchronoscope\  \ 


y 


\  Field  Rheostat 
\  Controller 


LT  'Voltmeters 


N23    ,'        N24         N25          N?6 


o  op 
•  o  • 
cjob 


o  oo 

o 
o  o  o 


go  p 
Oob 


o  oo: 

o    ; 

o  oo! 


H- Front  Elevation  of  Control  Desk  and  Instrument  Board 

Fia.  235. — Plan  of  face  of  desk  and  front  elevation  of  control  desk  and 
instrument  board. 


SBC.  13] 


SWITCHBOARD  AND  SWITCHGEAR 


257 


switchboard  proper,  as  shown  in  Figs.  230,  I  and  232.  Or  it 
can  be  carried  on  a  specially  designed  pipe  frame  (Fig.  233) 
back  of  the  board.  Where  the  voltage  is  relatively  high  and 
the  bus  structure  should,  therefore,  be  located  in  rooms  or 
chambers  distant  from  the  switchboard,  the  circuit-breakers 
can  be  mounted  in  a  masonry  structure  as  suggested  in  Fig. 
234. 


FIG.  236. — Remote-control  oil  switch  in  masonry-cell  mounting. 

303.  Electrical  Remote -control  Switchboards  are  ordinarily 
used  only  in  the  largest  and  most  important  installations. 
The  small  switches  which  control  the  operation  of  the  large 
oil  power  switches  may  be  mounted  on  a  panel  switchboard, 
on  a  control  pedestal  (Fig.  200),  or  on  a  control  desk  (Figs. 
198  and  235).  The  instruments  are  mounted  on  the  upper 
part  of  the  board  where  a  panel  switchboard  is  used  or  on  an 
instrument  pedestal  (Fig.  201),  where  a  control  pedestal  is 

17 


258  CENTRAL  STATIONS  [ART.  303 

utilized.  With  a  control  desk  the  instruments  may  be 
mounted  on  posts  (Figs.  198  and  235)  or  on  a  vertical  upward 
extension  of  the  control  desk.  As  suggested  in  Art.  291,  on 
the  face  of  the  control  desk  is  arranged  a  miniature  bus  struc- 
ture. The  operation  of  the  small  switches  on  the  switch- 
board permits  current  to  flow  through  the  magnet  M,  Fig. 
236,  of  the  electrically  operated  remote-control  oil  switches, 
whereby  they  may  be  opened  or  closed  at  the  will  of  the 
operator.  In  general,  no  two  electrically  operated  remote- 
control  boards  are  alike  because  each  is  usually  designed  to 
satisfy  certain  specified  conditions  of  operation.  The  possible 
variations  in  design  are  almost  endless,  hence  cannot  be 
considered  here. 


SECTION  14 

CHARACTERISTICS  OF  ELECTRIC  GENERATING 
STATIONS 

304.  The  Procedure  Which  Will  Be  Followed  in  Describing 
Electrical  Energy  Generating  Stations  is  this:  First,  certain 
general  considerations  relating  to  all  generating  stations,  re- 
gardless of  the  types  of  their  prime  movers,  will  be  treated. 
Second,  the  adaptability  of  each  of  the  different  classes  of 
prime  movers:  (a)  steam,  (6)  internal-combustion  engine,  and 
(c)  hydraulic,  will  be  described.     Third,  stations  having  steam 
prime  movers  will  be  studied  in  some  detail.    Fourth,  stations 
having  internal-combustion  engine  prime  movers  will  be  con- 
sidered.   Fifth,  hydro-electric  stations  will  be  examined. 

305.  In  Determining  the  Cost  per  Unit  of  Electrical  Energy 
Generated  by  a  Station  a  number  of  factors  must  be  included. 
Among  these  may  be:  (1)  cost  of  fuel,  if  any;  (2)  labor  cost  of 
attendance  and  operation;  (3)  cost  of  supplies,  such  as  oil  and 
waste;  (4)  interest  on  the  investment;  (5)  depreciation;  (6) 
taxes;  (7)  insurance;  and  (8)  repairs.     How  all  of  these  factors 
may  be  recognized  in  estimating  the  total  cost  is  indicated  in 
the  example  just  following.     It  is  obvious  that  each  specific 
problem  must  be  considered  on  its  own  merits.     The  reasons 
for  this  are  that  the  efficiencies  of  the  apparatus  involved,  the 
cost  of  fuel,  the  cost  of  attendance  and  similar  elements,  will 
vary  widely  under  different  conditions. 

EXAMPLE  OF  METHOD  OF  DETERMINING  COST  OF  GENERATING  ENERGY. 
— The  following  examples  are  quoted  to  illustrate  the  general  procedure 
and  the  principal  factors  involved  rather  than  to  emphasize  specific 
values.  The  problem  is  this:  Which  would  be  more  economical  under  the 
conditions  to  be  recited,  to  continue  to  operate  a  steam  plant  or  to  purchase 
electrical  energy  from  a  central  station?  The  connected  load  is  275  kw. 
The  maximum  demand  (maximum  load)  is  230  kw.  The  annual  energy 
consumption  is  62,700  kw.-hr.  There  are  two  generators  in  the  plant,  each 
driven  by  its  oum  steam  engine.  No.  1  it  a  200-fcu>.  unit,  NO,  2  is  a  75- 
269 


260  CENTRAL  STATIONS  [A*T.  305 

kw,  unit.  The  charge  which  the  central  station  would  make  would  be  based 
on  (1)  a  "demand"  or  "readiness-to-serve"  charge  of  $2  per  month  per  kw. 
of  connected  load  and  (2)  an  additional  "energy"  charge  o/0.9  cts.  ($0.009) 
per  kw.-hr.  consumed.  All  of  the  connected  apparatus  is  direct  current, 
hence,  if  central-station  energy  is  purchased,  it  must  be  direct  current  or  be 
converted  into  direct  current  for  utilization. 

SOLUTION. — The  following  comprises  the  solution,  submitted  by  H. 
Berkeley  Hackett,*  to  the  above  example.  Owing  to  the  values 
submitted  for  maximum  demand  and  monthly  energy  consumption,  it 
would  be  erroneous  to  assume  the  lO-hr.-day  service  during  26  days  per 
month,  which  is  the  customary  working  period  in  manufacturing  indus- 
tries, since  on  this  basis  the  average  hourly  load  would  exceed  the  max- 
imum demand.  In  order,  however,  to  reach  any  conclusion  it  is  necessary 
to  decide  upon  a  daily  operating  period,  consequently  it  will  be  assumed 
that  the  plant  in  question  operates  continuously  during  365  days  per 
year.  While  this  assumption  may  not  represent  actual  conditions,  it 
will  at  least  afford  a  basis  for  demonstrating  the  method  of  computing 
the  yearly  cost  of  generating  energy  and  comparing  same  with  central- 
station  service. 

The  first  step  will  be  to  find  the  boiler  capacity  required  to  meet  the 
peak  load  conditions,  in  order  that  fixed  charges  on  these  units  may  be 
properly  accounted  for.  With  a  maximum  demand  of  230  kw.,  the  prob- 
able "water  rate"  of  the  large  engine  will  be  40  Ib.  of  steam  per  kw.-hr., 
consequently  the  hourly  steam  consumption  under  these  load  conditions 
is:  230  X  40  =  9,200  Ib.  plus  10  per  cent,  for  auxiliaries,  pipe  line  losses, 
etc.,  or  a  total  of  10,120  Ib.  per  hr.  that  the  boilers  must  supply  during 
maximum-demand  periods. 

Assuming  that  the  steam  pressure  is  130  Ib.  gage  and  that  the  temper- 
ature of  the  feed  water  entering  boilers  is  200  deg.,  each  pound  of  feed 
water  must  receive  in  the  boilers  1024.8  heat  units  to  convert  into  steam 
at  the  assumed  pressure.  Therefore,  the  boilers  must  be  capable  of 
furnishing:  10,120  X  1,024.8  =  10,371,000  heat  units  per  hr.  Since  a 
boiler  horse-power  is  equivalent  to  33,500  heat  units,  the  boiler  capacity 
required  will  be:  10,371,000  -^  33,500  =  310  h.p.  Three  150  h.p.  water- 
tube  boilers  will  therefore  be  considered  in  estimating  the  installation 
costs. 

Tne  second  step  in  the  calculation  will  be  to  determine  the  yearly  coal 
requirements.  With  a  monthly  consumption  of  62,700  kw.-hr.,  the  av- 
erage hourly  load  of  24-hr,  service  will  be:  62,700  -4-  24  X  30  =86  kw. 
approximately.  To  allow  for  load  fluctuations  on  engines  and  boilers, 
it  will  be  assumed  that  the  water  rate  of  the  engines  is  45  Ib.  of  steam  per 
kw.-hr.,  and  that  the  boilers  evaporate  6  Ib.  of  steam  per  Ib.  of  coal. 
This  evaporation  corresponds  to  51  per  cent,  boiler  efficiency,  assuming 
the  heat  value  of  the  fuel  to  be  12,000. 

*  ''CALCULATING  COST  OF  POWER  AB  GENERATED  BY  STEAM,  Electrical  Engineering, 
January,  1916,  p.  41. 


SEC.  14] 


ELECTRIC  GENERATING  STATIONS 


261 


On  a  basis  of  8,760  hr.  per  year,  the  total  yearly  steam  requirements, 
including  10  per  cent,  for  auxiliaries,  pipe-line  losses,  etc.,  will  therefore 
be:  86  X  45  X  8,760  X  1.1  =  37,291,000  Ib.,  and  since  each  pound  of 
coal  evaporates  6  Ib.  of  steam,  the  coal  necessary  to  evaporate  this  quan- 
tity of  steam  is:  37,291,000  -^  6  =  6,215,100  Ib.,  or  3,110  short  tons. 

Having  determined  the  boiler  capacity  and  the  yearly  coal  require- 
ments it  is  possible  to  approximate  the  yearly  operating  costs,  as  given 
in  the  following  tabulation: 

INSTALLATION  COST 


Apparatus 

Cost 

Sub-totals 

One  120-h  p.,  4-valve  engine  (erected) 

$2  600 

One  320-h.p.,  4-valve  engine  (erected)  
Foundations  
Three  150-h.p.  water-tube  boilers  (erected)  
Steel  stack  foundation  —  breeching  erected  
Feed-water  heater  —  boiler  feed  and  house  pumps. 
Piping  —  covering  separators,  tanks,  etc  

5,800 
750 
7,500 
2,000 
1,000 
4,000 

$23,650 

One  75-kw.  direct-current  generator  erected  
One  200-kw.  direct-current  generator  erected  
Switchboard  and  wiring  .  . 

1,100 
3,000 
1,700 

5,800 

Total  cost  of  plant  

$29,450 

GENERATING  COST 


Interest  on  $29,450  @  5  per  cent  

$1,470 

Depreciation  on  29,450  @  6  per  cent  
Taxes,  insurance,  etc.,  on  $29,450  @  1  per  cent.. 

1,770 
295 

$3,535 

Fuel,  3,110  tons  @  $3  
Labor  (5  operators)  

9,330 
4,600 

100 

Oil  waste,  etc  
Maintenance  and  repairs  

300 
600 

$14,930 

Total  annual  cost  of  generating  67,200  X  12  = 
752  400  kw.-hr.                 

$18,465 

Cost  per  kilowatt-hour,  in  cents  =  18,465  •*•  752, 
752.400  -.... 

2.46 

262 


CENTRAL  STATIONS 
COST  OP  CENTRAL-STATION  SERVICE 


[ART.  305 


The  connected  load  is  275  kw.  The  primary  charge  will  be  $2  per 
month.  The  average  monthly  energy  consumption  of  62,700  kw.-hr. 
will  be  at  the  rate  of  0.9  cts.  per  kw.-hr.  If  a  rotary  (synchronous) 
converter  is  installed,  the  customer  will  probably,  be  required  to  pay  the 
conversion  losses.  With  a  converter  efficiency  of  90  per  cent.,  the 
monthly  bill  charged  against  the  consumer  will  therefore  be :  62,700  •*•  0.9 
=  69,500  kw.-hr.  The  annual  cost  of  purchased  power  will  therefore 
be  about  as  follows: 

INSTALLATION  COSTS 


One  250-kw  rotary  converter. 

$3  000 

Switchboard.               

W0) 
1  700 

Total  installation  cost  .  .  . 

$4,700 

OPERATING  COSTS 


Interest,  depreciation,  etc.,  @  12  per  cent,  on 
$4,700  

$560 

Primary  charge  —  275  X  12  months  X  $2.   .  .    . 

6,600 

Current  charge  —  69,500  X  12  X  $0.009     

7,500 

Night  and  day  attendant  

1,700 

Oil,  waste,  repairs,  etc  

70 

Total  yearly  cost  of  69,500  X  12  or  834,000  kw.- 
hr 

$16  430 

Cost  per  kilowatt-hour  in  cents 

1.97 

In  the  above  estimates  no  consideration  has  been  given  the  question  of 
heating.  Should  the  conditions  demand  this  requirement,  it  will  be  neces- 
sary to  make  certain  additional  charges  against  the  central-station  esti- 
mate in  order  to  obtain  a  true  comparison  of  power  costs.  These  charges 
would  consist  of  interest  and  depreciation  on  boiler  capacity  for  heating, 
corresponding  to  the  boiler  capacity,  available  in  the  private  plant  for 
this  purpose.  Also,  a  charge  for  fuel  for  generating  steam,  equivalent  in 
amount  to  the  exhaust  steam  from  the  engines  of  the  private  plant  that 
could  be  utilized  in  the  heating  system.  There  would  be  a  further  charge 
for  labor,  maintenance,  supplies,  etc.,  in  connection  with  the  operation 
of  boilers  for  heating. 


SEC.  14] 


ELECTRIC  GENERATING  STATIONS 


263 


The  following  table  illustrates  a  logical  and  systematic  method  of 
tabulating  the  fixed-charge  data  when  an  energy-cost  determination  is 
being  made.  This  table  shows  the  values  assumed  by  T.  B.  Hyde  in 
his  solution  of  the  example  above  proposed.  For  this  complete  solution 
refer  to  the  number  of  the  magazine  cited  in  the  footnote. 

STEAM  PLANT  (Energy  Generated) 


B 

Equip- 
ment 

Number  O 

D 

E 
Unit 

F 

Unit 
cost 

Q 

Total 
cost 

Fixed  charges  (per  cent.) 

N 

Fixed 
charges 
per  year 

Interest  -H 

Deprecia-  v, 
tion 

Insurance  >) 
taxes 

Main- 
tenance t" 
(repairs) 

M 

1 

Boilers  
Engine.  .  .  . 
Engine.  .  .  . 
Generator 
Generator 
Building... 
Feedpump 
Feed  -water 
Heater 
Piping 

3 

1 
1 
1 
1 

150 
320 
120 
200 
75 

B.H.P. 
I.H.P. 
I.H.P. 
K.W. 
K.W. 

$14.00 
11.00 
10.00 
12.50 
13.35 

$6,300 
3,520 
1,200 
2,500 
1,000 
9,000 
200 
600 

2,000 

5 
5 
5 
5 
5 
5 
5 
5 

5 

5 
5 
5 
5 
5 
3 
7 
5 

7 

1 
1 
1 
1 
1 
1 
1 

1 

3 
3 

1 
1 
3 
3 

3 

18 
14 
14 
12 
12 
10 
16 

14 
16 

$1,134 
493 
168 
300 
120 
900 
32 

84 
320 

400 

H.P. 

1.50 

Total 


3,551 


ELECTRIC  PLANT  (Energy  Purchased) 


Building 

$3,000 

5 

3 

1 

1 

10 

$300 

Synchron- 
ous 

2,000 

5 

5 

1 

12 

240 

Total.... 

$540 

NOTE. — DEPRECIATION  OF  THE  EQUIPMENT  OF  AN  ELECTRIC  PLANT.* — 
In  general,  plant  and  sub-station  buildings  may  be  assumed  to  have  a 
useful  life  of  about  50  years,  making  the  average  depreciation  about  2 
per  cent.  To  boilers,  piping,  generators,  electrical  equipment,  etc., 
lives  of  20  years  are  assigned.  Of  these,  boilers,  piping  and  generators 
have  net  salvage  values  of  4  to  6  per  cent,  at  the  end  of  that  time,  thus 
making  their  rate  of  depreciation  from  4.7  to  4.8  per  cent,  per  annum. 
Allowing  10  per  cent,  salvage  value  for  electrical  equipment,  the  rate  of 
depreciation  becomes  4.5  per  cent.  Storage  batteries,  with  a  useful 
life  of  20  years,  go  out  of  service  with  a  salvage  value  of  17  per  cent,  mak- 

•  Electrical  World. 


264  CENTRAL  STATIONS  [ART.  306 

ing  the  net  depreciation  4.15  per  cent,  per  year.  Poles  and  pole-line 
equipment  are  assigned  values  of  12  per  cent,  at  the  end  of  20  years' 
service,  resulting  in  a  depreciation  rate  of  4.4  per  cent.  Wire,  after  16 
years'  estimated  usefulness,  has  the  high  scrap  value  of  40  per  cent., 
making  the  depreciation  rate  3.75  per  cent.  Line  transformers  and 
customers'  meters  may  be  assumed  to  have  the  same  life  as  the  other 
electrical  equipment  named,  20  years,  but  at  the  end  of  that  time  they 
have  a  salvage  value  of  10  per  cent.,  making  the  net  depreciation  rate  in 
the  case  of  these  instruments  4.5  per  cent. 

306.  The  Location  of  a  Generating  Station  is  a  thing  which 
should  be  considered  most  carefully,  because  if  the  station  is 
not  located  intelligently  the  cost  of  the  energy  generated  and 
delivered  by  it  may  be  excessively  high.     In  this  connection 
the  following  items*  are  of  importance:  (1)  accessibility.  (2) 
coal  and  water  supply,  (3)  stability  of  foundation,  (4)  facilities 
for  extension,  (5)  cost  of  real  estate  and  taxes,  and  (6)  situa- 
tion should  be  such  that  the  output  of  the  plant  may  be  effect- 
ively utilized. 

NOTE. — The  station  should,  all  things  being  equal,  be  easily  accessible 
so  as  to  facilitate  the  delivery  of  fuel,  stores  and  machinery,  while  it 
should  be  so  located  that  the  ashes  may  be  easily  removed.  If  possible, 
the  station  should  be  so  located  that  it  may  be  reached  by  both  rail 
and  water. 

307.  The  Advantages  of  Centralization,  that  is,  the  advan- 
tages which  accrue  through  the  concentration  of  generating 
equipment  into  one  large  plant  rather  than  having  it  scattered 
among  a  number  of  small  plants  may  be  recited  thus:*  (1) 
It  is  possible  to  distribute  the  power  economically,  (2)  because 
of  the  diversified  nature  of  the  load  it  is  possible  to  operate  the 
system  with  a  better  load  factor,  (3)  purchasing  supplies  and 
spare  parts  at  a  central  point  is  a  decided  economic  advantage, 
(4)  the  centralization  of  management  and  operating  force  re- 
duces the  overhead  expenses,  (5)  it  is  possible  to  serve  certain 
classes  of  customers  whom  the  small  individual  plant  could 
not  afford  to  serve,  (6)  the  lower  cost  of  production,  due  to 
centralized  service,  makes  it  possible  to  offer  lower  rates,  (7) 
consolidation  of  interests  makes  possible  the  financing  of  im- 

*  THE  LOCATION  OF  POWEB  PLANTS,  by  J.  M.  Kearna  of  the  Boston  Ediaon  Company. 


SEC.  14]  ELECTRIC  GENERATING  STATIONS  265 

provements,  of  substitution  of  new  for  obsolete  apparatus  and 
the  extension  of  service  into  new  territory  and  (8)  better  regu- 
lation and  better  protection  can  be  provided. 

308.  Direct-current  Voltages  and  Systems  may  be  divided 
into  two-wire  and  three-wire  systems.     Two-wire  systems 
(which  are,  ordinarily,  desirable  only  for  electric-lighting  ser- 
vice where  the  power  is  transmitted  over  very  short  distances) 
usually  operate  at  110  volts  because  110  volts  is,  due  to  eco- 
nomic reasons,  the  most  desirable  pressure  for  operating  incan- 
descent lamps  in  multiple.     Occasionally,  in  a  direct-current 
installation,  where  the  power  must  be  transmitted  for  a  dis- 
tance of  possibly  something  under  a  mile  for  direct-current 
motors,  a  220-volt  direct-current  system  may  be  installed. 
If  this  is  done,  220-volt  incandescent  lamps  are  used.     It  is 
seldom  that  a  two-wire  direct-current  system  is  now  installed 
for  general  electric  lighting  and  power  service.     Usually,  be- 
cause of  the  economics  which  result  therefrom,  a  three-wire 
system  is  used.     The  pressure  between  the  outer  wires  is  220 
volts  and  between  the  neutral  and  each  of  the  outers,  110 
volts.     With  this  system,  the  incandescent  lamps  can  operate 
on  110  volts  and  the  motors  on  220.     The  stations  in  many 
office  buildings,  industrial  plants  and  small  towns  generate  on 
110-220- volt  three-wire  system.     For  urban  railway  service, 
the  standard  direct-current  pressure  is  600  volts.     Pressures 
as  high  as  1,200,  2,400,  1,500  and  3,000  volts  have  been  applied 
recently  for  interurban  and  trunk  line  railway  service. 

308A.  The  Generation  of  Alternating  Voltages  is  now,  in 
stations  serving  large  cities  or  considerable  loads  of  any  char- 
acter, the  usual  practice — because  of  the  economies  which 
result  therefrom.  Then,  if  at  any  location  on  the  distribution 
system,  direct  current  is  required,  the  alternating  can  be  con- 
verted into  direct  voltages  by  using  motor-generators  or  syn- 
chronous converters.  A  large  percentage  of  all  generating 
stations  which  have  been  installed  recently — this  includes 
many  small  stations  as  well  as  large  ones — generate  only  alter- 
nating voltages. 

309.  Practically  All  Alternating-current  Stations  Generate 
Three-phase. — This  refers  to  stations  which  have  been  in- 


266 


CENTRAL  STATIONS 


[ART.  310 


stalled  recently.  (There  are  still  a  number  of  two-phase  stations 
and  a  very  few  single-phase  stations  in  operation.)  The 
reason  for  this  is  that  three-phase  generating  and  converting 
apparatus  is  the  most  economical  and  energy  can  be  trans- 
mitted more  economically  with  the  three-phase  system  than 
with  any  of  the  others  which  are  utilized  directly.  The  three- 
phase  can  be  readily  transformed  or  converted  into  power  of 
some  one  of  the  other  systems  if  desirable. 

NOTE. — Even  if  a  large  proportion  of  the  output  of  a  station  must 
be  transformed  or  converted  to  render  it  available  for  utilization,  it  is 
usually  most  economical  to  generate  only  one  kind  of  energy.  That  is, 
for  most  cases  in  a  station  of  any  size,  only  three-phase  alternating  power 
should  be  generated.  The  reason  why  this  plan  is  ordinarily  followed  is 
that  with  it  the  generating  units  can  all  be  operated  at  greater  loads. 
That  is,  the  individual  load  factors  of  the  generating  units  can  be  main- 
tained at  a  maximum,  due  to  the  ad- 
vantage that  may  thereby  be  taken 
of  the  diversity  element.  Further- 
more, where  only  one  kind  of  power 
is  generated,  the  investment  which 
must  be  tied  up  in  reserve  apparatus 
may  be  a  minimum. 

310.  Three-phase  Generators 
are  Usually  Star-connected  (Fig. 
237),  because  with  this  method 
of  connection  for  a  given  voltage, 
EL  (Fig.  237)  between  line  wires, 
the  voltage  Ec  across  each  set 
of  armature  coils  is  less  than  it 
would  be  with  a  delta-connected 
generator  impressing  the  same 
line  voltage.  In  view  of  this, 
the  star-connected  generator 


--Connection 


FIG.  237. — Diagrammatic  repre- 
sentation of  a  star-  or  Y-connected. 
three-phase  generator. 


the  number  of  turns  in 
coils  is,  for  a  given  line  voltage,  smaller.  Hence,  con- 
ductors of  a  correspondingly  larger  diameter  can  be  used 
for  them  than  would  be  necessary  with  a  delta-connected 
machine.  Thus  a  more  sturdy  and  satisfactory  mechanical 

*  Much  of  the  material  which  folllows  is  based  on  data  contained  in  the  article 
GENERATING  STATIONS  AND  SOME  FEATURES  GOVERNING  THEIR  DESIGN  by  E.  A.  Lof 
o'f  the  General  Electric  Company,  published  in  Coal  Agt,  F«b.  6,  1916. 


SEC.  14]  ELECTRIC  GENERATING  STATIONS  267 

structure  results.  Furthermore,  certain  difficulties,  due  to 
local  current  circulating  in  the  machine  windings,  which  may 
occur  with  the  delta  are  eliminated  with  the  star  connection. 
311.  Both  Grounded  and  Ungrounded-neutral  Systems  are 
used.  Which  is  preferable  must  be  determined  in  each  specific 
case  after  due  consideration  has  been  given  to  the  factors  in- 
volved. There  are  two  reasons  for  grounding  the  neutral. 
One  is  to  limit  the  voltage,  between  line  and  ground,  which 
may  be  impressed  on  the  insulators  and  apparatus. 

.-Neutral  Point  \\ 


~* —  — * — r 

'•i/ne          :      r 
•     Wfres^      .L      •: 


J  -Phase  Generator  ••   Insulator- 


rrifr-*f*rw.        If^dlHTr;.        ;,V;- 

G  "?V  Permanent  Insulator-    •  •?. •':•=-  Accidental  ^ff,.  "  .'• 

'Ground  Orpund— •*=  v" 

Fio.  238. — Accidental  ground  on  a         FIG.  239. — Accidental-ground  on  an 
grounded-neutral  system.  ungrounded-neutral    system. 

EXPLANATION. — If  a  neutral  is  grounded,  as  at  G,  Fig.  238,  it  is  evident 
that  the  electric  stress  imposed  on  the  insulator  at  7,  between  line  wire 
L3  and  ground,  can  never  be  greater  than  the  voltage  EC  which  is  0.58 
of  the  line  voltage,  E.  If,  however,  the  neutral  is  ungrounded  as  in 
Fig.  239,  and  an  accidental  ground,  GA,  occurs,  then  the  line  voltage  E  is 
impressing  on  the  insulator  at  /»  between  the  line  wire,  C,  and  ground. 
The  line  voltage  in  a  three-phase  system  always  equals  1.73  times  the 
voltage  to  neutral.  (The  voltage  to  neutral  is  shown  by  EC  in  Fig.  237.) 

The  other  reason  for  grounding  a  neutral  is  to  insure  that 
one  of  two  or  more  sets  of  feeders  will,  by  the  action  of  the 
overload  oil  circuit-breaker  inserted  in  it,  promptly  disconnect 
itself  from  the  station  if  a  ground  occurs  on  that  feeder. 

EXPLANATION. — If  an  accidental  ground,  G2  (Fig.  240),  occurs  on  a 
feeder,  FI,  supplied  by  a  generator  having  a  grounded  neutral,  a  current- 
will  flow  through  the  accidental  ground  as  shown  by  the  dotted  arrows. 
This  ground  will  be  of  sufficiently  low  resistance  so  that  the  current  which 
flows  will  be  of  great  enough  intensity  (amperage)  to  immediately  operate 
the  automatic  oil  switch  or  oil  circuit-breaker,  Si.  This  will  isolate  F\ 
from  the  system.  If  the  neutral  were  not  grounded  the  attendants  in 
the  station  where  the  generator  G  was  located  might  not  be  informed 
promptly  of  the  existence  of  the  accidental  ground,  G> 


268 


CENTRAL  STATIONS 


[ABT.  312 


It  follows  that,  if  only  one  feeder  extends  from  a  station  to 
a  given  load  and  that  continuous  service  is  essential,  grounding 
is  probably  undesirable.  If  on  the  other  hand,  two  or  more 
feeders  extend  from  the  station  to  the  same  load  then  ground- 
ing may  be  desirable.  As  a  rule  it  is  good  practice  to  omit  the 
grounding  of  generators  where  only  one  or  two  feeders  extend 
from  the  station  unless  the  system  operates  at  a  very  high 
voltage.  Where  the  neutral  is  grounded  merely  to  insure  the 
automatic  disconnection  of  a  feeder  in  the  event  of  a  ground 
on  it  (Fig.  240),  a  resistance,  R,  may  be  inserted  between  the 


Automatic  Oil' 
'Circuit  Breakers 


••Generator  Neutral 
Permanently  Grounded 


-Path  of  Short- 
Circuit  Current 


- 

AccfrenW 
Ground. 


FIG.  240. — Showing  how  an  accidentally-grounded  feeder  will   disconnect 
itself  from  the  generator  on  a  grounded-neutral  system. 


neutral  point  of  the  generator  and  ground,  to  prevent  the  flow 
of  excessive  current  in  case  of  an  accidental  ground,  G2,  on 
the  line.  Such  a  resistance  should  be  so  proportioned  that  it 
would  permit  enough  current  to  flow  to  operate  the  automatic 
oil  switches  but  would  at  the  same  time  prevent  the  flow  of  a 
dangerously  large  current. 

312.  The  Voltages  for  Alternating-current  Generators  vary 
with  the  conditions  under  which  the  plant  operates.  Occa- 
sionally where  a  very  small  plant  serves  only  an  incandescent- 
lighting  load,  the  generator  voltage  is  110.  More  frequently 
a  pressure  of  220  is  adopted  for  small  plants  which  serve  a 
three- wire  system,  the  three-wire,  110-220- volt  pressures 
(Fig.  241)  being  obtained  with  balance  coils  located  at  points 
near  the  load.  Sometimes,  in  industrial  plants  where  a  con- 
siderable number  of  alternating-current  motors  are  used,  480- 


SEC.  14] 


ELECTRIC  GENERATING  STATIONS 


269 


'  Alternating-Current 


Municipal  Strett  tigfrfing 

,Watthour  Meter  •^Commercial  and  Residence 

'  Totalizing 


Incandescent Lamps  --.'.'_. ; 

FIG.  241. — General  scheme  of  generation  and  distribution  for  a  town    of 
about  1,000  inhabitants  or  less. 


Fia.  242. — Arrangement  of  the  generating  and  electrical  equipment  in  a 
modern  steam-turbine  generating  station.* 

•  E.  A.  Lof,  in  COAL  AGE,  Feb.  6,  1915. 


270 


CENTRAL  STATIONS 


[ART.  313 


volt  three-phase  generators  are  installed.  Generators  for. 
2,200  to  2,400  volts  three-phase  are  very  frequently  used  in 
towns  and  small  cities  for  serving  a  general  lighting  and  motor 
load  In  large  industrial  plants  three-phase  6,600-volt  gen- 
erators are  ordinarily  used.  A  pressure  of  13,200  volts  is  the 
highest  for  which  it  is  deemed  desirable  in  the  United  States 
to  wind  alternating-current  generators.  Hence,  a  great  many 
machines  generate  at  this  pressure.  Where  the  transmission 
distance  is  sufficiently  short  it  is  economical  to  use  13,200  as 


-Star -Connected  Generator 


Neutral  Wire  Grounded  I 
at  Intervals  Along  Line 


<.-6rounti  Connection 


FIG.  243. — The  four-wire,  three-phase  system. 

the  transmission  voltage.  In  many  plants  the  generation 
voltage  is  2,300  or  2,400  (Fig.  242)  and  this  pressure  is  raised 
with  transformers  to  one  suitable  for  the  long-distance  trans- 
mission. The  four-wire,  three-phase  system  (4,000  volts 
between  line  wires  (Fig.  243)  and  2,300  volts  to  neutral)  is 
now  being  used  extensively  for  local  distribution  for  light  and 
power  in  cities.  In  any  case  the  generator  voltage  which 
should  be  selected  is  largely  a  matter  of  economics. 

313.  The  Selection  of  the  Capacities  and  Ratings  of  the 
Generators  is  a  question  which  is  always  worthy  of  careful 
consideration.  As  a  rule  in  modern  installations  each  genera- 
tor and  its  prime  mover  constitutes  a  complete  unit.  It  is 
almost  always  the  best  practice  to  install  two  or  more  such 
units  in  every  plant  so  that  it  will  be  possible,  under  all  ordi- 
nary operating  conditions,  to  work  at  least  one  of  the  units  at 
its  most  efficient  load;  hence,  in  determining  capacities  and 
ratings,  it  is  always  desirable  to  baae  the  determination  upon 


SEC.  14]  ELECTRIC  GENERATING  STATIONS  271 

the  graph  of  the  load  to  be  imposed  on  the  station.  It  is, 
however,  seldom  possible  to  base  the  selection  solely  on  the 
load  graph  because  provision  should  be  made  for  a  certain 
growth  in  load.  Also,  the  question  of  reserve  capacity — that 
is,  protection  against  accident  and  breakdown — should  be 
considered. 

314.  The  Rating  of  the  Generator  Should  Be  Proportioned 
to  the  Characteristics  of  the  Prime  Mover. — Both  steam  en- 
gine and  waterwheel  prime  movers  operate  at  maximum  effi- 
ciency at  certain  definite  loads.     At  loads  greater  or  smaller 
than  this  maximum-efficiency  load  there  will  be  a  material 
decrease  in  efficiency.    With  internal-combustion  engine  prime 
movers  of  this  type  have  no  overload  capacity)  the  point 
of  greatest  efficiency  is  the  maximum  load  which  the  prime 
mover  will  pull.     These  elements  should  be  given  due  consi- 
deration. 

315.  In  Providing  Reserve  Capacity  in  generating  equip- 
ment, that  is,  capacity  which  may  be  utilized  in  case  of  a  break- 
down of  some  of  the  generating  apparatus,  there  are  two  ex- 
pedients which  may  be  adopted.     That  is,  reserve  capacity 
may  be  provided  by:  (1)   installing  an  extra  unit  or  extra 
units  which  may  be  operated  when  one  of  the  regular  units  is 
inoperative,  and  (2)  the  overload  capacity  of  the  regular  units. 

NOTE.* — With  the  method  of  rating  engine-driven  units  which  is  gen- 
erally used  which  provides  a  liberal  overload  capacity  for  a  few  hours, 
the  second  expedient  works  out  satisfactorily.  For  example,  a  plant 
of  five  units,  each  of  which  has  an  overload  capacity  of  25  per  cent., 
can  have  one  unit  taken  out  of  service  when  all  of  the  units  are  operating 
at  full-load  without  placing  an  excessive  load  on  the  remaining  units. 
It  is  now  the  almost-standard  practice  of  practically  all  steam-turbine 
manufacturers  to  rate  their  units  on  a  maximum  basis,  that  is,  without 
any  overload  capacity.  Under  this  method  of  rating  it  is  necessary  to 
utilize  the  first  expedient  noted  above  to  provide  reserve  for  emergencies. 
To  maintain  the  reserve  capacity  at  a  minimum  it  may  be  desirable  to 
have  at  least  five  or  six  units  in  the  plant.  With  five  units  to  carry  the 
load  and  one  reserve  unit  there  is  then  only  17  per  cent,  of  the  installa- 
tion held  in  reserve.  If  one  reserve  unit  is  considered  inadequate  the 
addition  of  another  increases  the  reserve  capacity  to  about  29  per  cent, 
of  the  total. 

•J.  W.  Shuster,  TENDENCIES  IN  CENTRAL  STATION  PRACTICE,  Electrical  Review, 
Mar.  3,. 1917. 


272  CENTRAL  STATIONS  [ART.  316 

Waterwheel  and  internal-combustion  engine  driven  units 
have  no  overload  capacity,  hence  in  stations  driven  by  prime 
movers  of  this  type  reserve  capacity  must  be  provided  as 
outlined  in  expedient  (2),  above.  That  is,  additional  units 
must  be  provided  for  reserve. 

316.  Low  Power  Factor  Decreases  the  Effective  Capacity 
of  a  Generator  so  that  if  a  station  is  to  operate  at  a  power 
factor  other  than  100  per  cent,  this  should  be  recognized. 

EXAMPLE. — If  the  power  factor  of  a  plant  is  100  per  cent,  then  for  a  100- 
kva.  generator,  in  such  a  plant,  a  prime  mover  capable  of  developing 
approximately  100  kw.  output  should  be  provided  for  the  generator. 
However,  if  the  power  factor  of  the  load  which  the  plant  serves  is  only 
75  per  cent.,  then  to  drive  a  100-kva.  generator  a  prime  mover  of  only 
approximately  75  kw.  output  is  necessary.  If  a  100-kw.  prime  mover 
were  used  approximately  25  per  cent,  or  one-fourth  of  its  capacity  would 
be  unavailable.  Furthermore,  the  prime  mover,  instead  of  being  fully 
loaded,  that  is,  operating  at  high  efficiency  when  the  generator  was 
fully  loaded,  would  be  operating  at  only  three-fourths  load  with  corre- 
spondingly low  efficiency.  (In  the  preceding  example  a  generator 
efficiency  of  100  per  cent,  has  been  assumed.)  Actually  the  efficiency 
of  a  generator  is  always  less  than  100  per  cent.,  but  the  principle  involved 
is  evident  from  the  above  even  if  the  generator  efficiency  is  not 
considered. 

317.  The  Unit  Principle  Should  be  Utilized  wherever  pos- 
sible.    That  is,  all  of  the  units  in  the  plant  should,  insofar  as 
feasible,  be  duplicates.     The  different  essential  elements  of 
the   installation,    such   as   generating   elements    (comprising 
generator  prime  mover  and  its  auxiliary  apparatus)  trans- 
formers, boilers,  etc.,  should  be  arranged  in  groups  or  composite 
units.     Each  one  of  these  groups  should,  in  essence,  be  a  com- 
plete central-station  installation  in  itself.     Then,  if  this  plan 
is  followed,  a  breakdown  in  some  component  piece  of  apparatus 
should  effect  only  the  group  of  which  that  component  forms 
a  part.     The  other  groups  should  be  capable  of  uninterrupted 
operation. 

318.  The  Factors  Which  Should  Determine  the  Location 
of  the  Apparatus  in  a  generating  station  are:  (1)  simplicity, 
(2)  reliability  of  operation  and  (3)  extensions.     Upon  study 
it  will  be  evident  that,  in  general,  the  unit  system  of  arrange- 
ment described  in  the  preceding  article  satisfies  all  of  these 


SEC.  14]  ELECTRIC  GENERATING  STATIONS  273 

requirements     more    completely    than    would    any    other 
arrangement. 

319.  In  Locating  the  Prime  Movers  and  Generators  they 
should  ordinarily  be  arranged  on  the  main  floor  of  the  station. 
There  should  always  be  at  least  sufficient  distance  between 
units  to  admit  a  free  passage  around  them  and  so  that  repairs 
can  be  effected  without  any  unnecessary  waste  of  time.     In 
steam  plants  the  condensers  are  usually  located  in  the  base- 
ment and  the  principal  piping  (Fig.  242)  carried  below  the 
main  floor. 

320.  The  Exciters*  should  have  a  capacity  sufficient  for 
all  of  the  synchronous  apparatus  in  the  station  when  all  of  the 
synchronous  machines  are  operating  at  their  maximum  loads 
and  at  the  operating  power  factor.     It  is  not  sufficient  to 
provide  only  enough  exciter  capacity  for  excitation  for  the 
machines  when  they  are  running  at  unity  power  factor.     The 
excitation   required    for   power  factors  lower  than  unity  is 
considerably  higher  than  that  required  at  unity. 

321.  The  System,  of  Excitation  which  is  now  considered 
good  practice  and  which  affords  maximum  reliability  is  that 
in  which  all  of  the  exciting  current  is  obtained  from  a  common 
source.     This  common  source  should  comprise  as  few  units 
as  possible.     One  or  two  units  are  generally  provided  for 
normal  excitation  and  a  third  is  installed  as  a  reserve.     It  is 
always  good  practice  to  have  the  regular  exciter  driven  by 
prime  movers  such  as  steam  engines  or  waterwheels,  while 
the  reserve  unit  is  motor-driven. 


NOTE. — Another  system  of  excitation  which  is  now  being  used  fre- 
quently is  to  install  (for  driving  the  exciters)  low-voltage  generators 
each  of  which  is  driven  by  a  non-condensing  steam  turbine  or  a  water- 
wheel.  The  exciters  are  then  motor-driven,  energy  therefor  being  ob- 
tained from  the  low-voltage  generator.  The  steam  from  the  turbines 
exhausts  into  the  feed-water  heaters.  In  addition  to  the  exciters,  all  of 
the  other  auxiliaries,  such  as  circulating  pumps,  etc.,  are  motor-driven. 


»  E.  A.  Lof.  of  the  General  Electric  Company,  GENERATING  STATIONS  AND  SOMI 
FEATURES  COVERING  THEIR  DESIGN"  published  in  Coal  Age,  Feb.  5,  1915. 

18 


274  CENTRAL  STATIONS  [ART.  322 

322.  The  Exciter  Voltage  is,  for  small  and  medium-sized 
plants,  125  volts.     For  large  installations  250-volt  excitation 
is  more  economical.  • 

323.  In  Locating  the  Exciters  in  a  station  it  is,  in  general, 
desirable  to  place  them  near  the  center  of  the  generator  room 
so  that  the  excitation  wiring  will  involve  minimum  cost. 

NOTE. — Where  one  exciter  is  furnished  for  each  generator  it  should 
be  located  as  close  as  is  feasible  to  its  generator.  Exciters  direct-con- 
nected to  their  generators  are  often  used. 

324.  Automatic  Voltage  Regulators  are  always  installed  in 
modern  stations.     These,  by  acting  on  the  fields  of  the  exciters, 
maintain  the  alternating-current  voltage  constant  at  the  bus- 
bars regardless  of  changes  (within  reasonable  limits)  of  load 
on  station  or  of  changes  in  prime-mover  speed.     Voltage  regu- 
lators are  usually  located  on  or  near  the  switchboard. 

325.  The  Number  and  the  Capacities  of  the  Transformers 
should  be  determined  by  the  characteristics  of  the  station  and 
the  load  which  it  serves.     In  stations  transmitting  at  medium 
or  low  voltage  it  is  usually  considered  best  practice  to  install 
one  bank  of  transformers  for  each  generating  unit  following 
out  the  unit  principle  recommended  above.     However,  where 
the  transmission  voltage  is  high,  the  transformer  bank  should 
form  a  unit  with  the  transmission  line,  each  of  the  transmission 
lines  terminating  in  the  station  in  one  of  these  units.     Where 
this  arrangement  is  followed  switching  on  the  high-tension 
side  of  the  transformers  is  unnecessary  as  all  of  the  switching 
can  then  be  effected  on  the  low-tension  side  of  the  transformers. 
In  locating  the  transformers  in  the  station,  they  are  usually 
placed  (Figs.  242  and  244)  on  the  main  floor  back  of  the  gener- 
ators.    They  are  enclosed  in  suitable  fireproof  compartments. 
A  track  is  installed  along  in  front  of  the  tier  of  compartments 
so  that  the  transformers  may,  when  necessary,  be  readily  re- 
moved for  repairs. 

326.  Either  Single-phase  or  Three-phase  Transformers  can 
be  used.     The  modern  tendency  appears  to  be  toward  the 
utilization  of  three-phase  transformers  where  the  application 
of  such  is  feasible. 


SEC.  14]  ELECTRIC  GENERATING  STATIONS 


275 


NOTE. — The  conditions  under  which  single-phase  transformers  are 
preferable  is  where  only  one  group  is  installed  or  where  the  expense  of 
a  spare  transformer  is  unwarranted.  ^In  such  installations  the  burning 
out  of  one  phase  of  a  three-phase  unit  involves  considerable  inconvenience 
since  the  transformer  would  have  to  be  disconnected  before  repairs  could 
be  made.  If  single-phase  transformers  are  used  and  connected  in  delta 
(on  both  primary  and  secondary)  the  damaged  unit  can  be  readily  cut  out 
and  the  other  two  operated  at  normal  temperature — 58  per  cent,  of  the 
rated  normal  capacity  of  the  group — until  the  damaged  unit  can  be 
replaced. 


FIG.  244. — Sectional    Elevation    of    a    typical    110,000-volt    Hydro-electric 
generating  station  * 

NOTE. — With  the  three-phase,  shell-type  transformers  both  the  pri- 
mary and  secondary  windings  are  delta-connected.  Trouble  in  one 
phase  will  not  prevent  the  use  of  the  other  two  in  open  delta.  By  short- 
circuiting  both  the  primary  and  secondary  of  the  damaged  phase  and 
cutting  it  out  of  the  circuit,  the  magnetic  flux  in  that  section  is  entirely 
neutralized.  Three-phase  transformers  may  be  used  in  moderate- 
voltage  installations  having  a  large  number  of  units.  For  high-voltage 
developments  where  each  transformer  bank  should  be  of  a  capacity  equal 
to  that  of  the  lines  which  it  serves  it  is  usually  necessary  to  select  single- 
phase  transformers  so  as  to  obtain  the  required  capacity  and  minimize 
the  cost  of  the  spare  unit. 

•  E.  A.  Lof  in  COAL  AGE,  Feb.  6.  1915. 


276 


CENTRAL  STATIONS 


[ART.  327 


327.  Transformers  May  Be  Either  of  the  Oil-cooled,  Water- 
cooled  or  the  Air-blast  Type. — In  the  oil-cooled  (Fig.  245)  or 
self-cooled  type  the  oil,  heated  by  contact  with  the  trans- 
former core  and  windings,  rises  to  the  top  in  the  transformer 


Cooling  Water 
•Coils 


Oil     •'      Heat  Radiating  ..-•' 
Valve        Pipes 

FIG.  245. — Oil-cooled  or  self-cool- 
ing transformer.  (This  shows  the 
tubular  type.) 


Oil-Draining  Valve 

FIG.  246. — The  water-cooled    trans- 
former. 


case,  from  which  the  heat  is  radiated  into  the  air.  The  tanks 
of  self-cooling  transformers  are,  therefore,  usually  made  of 
corrugated  sheet  steel  to  provide  maximum  radiating  surface. 
The  self-cooled  transformers  are  the  most  frequently  used. 
Where  water  for  cooling  purposes  is  available,  the  water-cooled 
transformer  (Fig.  246)  is  the  most  economical  in  first  cost. 
In  a  transformer  of  this  type,  the  oil  circulates  in  a  manner 
similar  to  that  in  a  self-cooled  unit.  However,  the  greater 


SEC.  14] 


ELECTRIC  GENERATING  STATIONS 


277 


proportion  of  the  heat  is  carried  away  by  the  water  forced 
through  a  pipe  coil  which  is  submerged  in  the  hottest  oil  in 
the  top  of  the  transformer.  Ordinarily  the  water  rate  is  ap- 
proximately Yz  gal.  per  min.  per  kw.  loss,  the  temperature  of 
the  incoming  water  being  59  deg.  F.  In  the  air-blast  trans- 
former (Fig.  247),  the  cooling  is  effected  by  forcing  a  blast  of 


Motor-Driven  Blower-. 


FIG.  247. — An  air-blast-transformer  installation. 


air  through  ducts  or  spaces  provided  in  the  transformer  struc- 
ture. Transformers  of  this  type  are  applicable  for  voltages 
up  to  about  33,000  but  they  are  being  rapidly  superseded  by 
those  of  the  self-cooling  type. 


NOTE. — Circulating-oil-type  transformers  (Fig.  248)  may  be  adopted 
where  the  only  water  available  for  cooling  is  hard  or  contains  sediment. 
Such  sediment  might  deposit  on  the  insides  of  the  coils  of  water-cooled 
transformer.  But  with  the  arrangement  shown  in  the  illustration, 
the  deposit  would  be  on  the  outside  of  the  oil  coils,  hence  readily  removed. 


278 


CENTRAL  STATIONS 


[ART.  328 


328.  External  Reactances  or  Reactors*  (Fig.  249)  are  now 
used  in  many  stations  of  large  capacity  to  limit  the  excessive 
current  which  would  now,  if  these  reactors  were  not  inserted,  in 


Conductors^ 


•terminals 


Clamping 


Oil 
Reserwir- 


FIG.  248. — The  circulating-oil-type         FIG.     249. — A    current-limiting    re- 
transformer,  actor. 


Feeders.  Feeder  Group  Bus-... 

Ft|F2|F3>jF4|Fs|F« 


.... .62 


Generators-' 


. 

R4^ 

TTyTfinr-Hi 

Ft  F?  FsV  F*  Fs  Fb  \  FT  Fa  Fa     Fio  FH  Fu 
Feeders'  '-Reactors 

FIG.      251. — Single-line     diagram 


'•Main  Sycfironiiing  Bus 

''Double  Reactors 

FIG.  250. — Single-line  diagram  of 
combined  generator,  bus  and  feeder-       showing  reactors  installed  on  feeder 
group  reactance  coil.  groups. 

the  case  of  a  short-circuit  on  the  system.     It  is  not  feasible  to 
incorporate  sufficient  reactance  in  high-voltage  turbo-gener- 

•  A.  I.  E.  E.  STANDARDIZATION  RULES. 

W.  H.  Dann  and  H.  H.  Rudd,  THE  USE  OP  CURRENT  LIMITING  REGULATORS,  Practical 
Engineer,  Aug.  15,  1915. 


SEC.  14] 


ELECTRIC  GENERATING  STATIONS 


279 


ators;  hence,  with  generators  of  large  capacity  of  this  type, 
current-limiting  reactances  may  be  inserted  in  the  generator 
lead  or  between  the  bus  sections  (Figs.  250,  251  and  252)  or 
in  the  outgoing  feeders. 

329.  The  Percentage  Reactance  of  a  Reactor*  is  the  ratio 
(expressed  in  per  cent.)  of  the  voltage  drop  across  it,  when 
full-load  current  flows  at  the  voltage  of  the  system. 

EXAMPLE. — If  the  full-load  current  is  100  amp.  (on  a  10,000-volt,  single- 
phase  system  or  on  a  three-phase  system  with  10,000  volts  to  neutral) 
the  drop  across  a  coil  having  a  reactance  of  5  ohms  will  be:  100  X  5  =- 
500  volts.  The  percentage  reactance  of  the  coil  will  be:  100  X  500  -J- 
10,000  =  5  per  cent.  Or  if  the  full-load  current  is  100  amp.  on  a  three- 


Fi    F?   Fa     F4     Fs    F&     Fi 
FIG.  252. — Reactors  inserted  in  generator  leads. 

phase  system,  with  10,000  volts  between  phases,  then  the  percentage 
reactance  of  the  coil  having  a  reactance  of  5  ohms  will  be:  100  X  500  •*• 
(10,000  X  0.577)  =  8.66  per  cent. 

The  short-circuit  current  which  will  flow  is:  100  -i-  (the  percentage  re- 
actance of  the  coil  +  the  percentage  reactance  of  the  generator)  X  full-load 
current.  In  the  above  case  if  the  generator  reactance  is  also  5  per  cent., 
the  short-circuit  current  will  be  10  X  full-load  current.  On  short-circuit 
the  reactance  will,  in  this  case,  have  one-half  the  no-load  voltage  im- 
pressed across  it. 

330.  The   Location  of  the   Switchboard   and   Switchgear 

should  be  determined  by  the  capacity,  voltage  and  general 
arrangement  of  the  plant.  In  low-voltage,  small-capacity 
plants  where  self-contained  switchboards  are  used,  all  of  the 
switchgear  is  mounted  directly  on  or  adjacent  to  the  switch- 
board. The  switchboard  is  installed  in  a  convenient  oentral 

•  Electrical  Journal,  Apr.  15,  1917. 


280  CENTRAL  STATIONS  [ART.  330 

location  near  one  of  the  walls  on  the  main  floor  of  the  station. 
Where  the  switchboard  is  of  the  remote-control  type  there  are 
many  different  arrangements  which  may  be  used.  It  is  usually 
considered  desirable  to  so  locate  the  control  board  that  an 
unobstructed  view  of  the  station  may  be  had  from  it.  How- 
ever, in  certain  very  large  installations  the  control  board 
is  in  a  room  entirely  separate  from  the  generating  room.  The 
bus-bars  and  oil  switches  are  located  on  the  various  floors  of 
the  switch  house.*  See  Figs.  242  and  244. 

•  E.  A.  Lof. 


SECTION  15 

ADAPTABILITY  OF  STEAM,  INTERNAL-COMBUSTION 
ENGINE  AND  HYDRAULIC  PRIME  MOVERS 

331.  The  Type  of  Prime  Mover  Which  Should  Be  Adopted 
for  Any  Specific  Installation  is,  in  general,  a  question  of 
economics.  Usually  that  prime  mover  is  the  best  one 


I9M 


1910        191? 


1906         1908 
Year 

FIG.  253. — Prime  movers  used  in 
central  stations.* 


1903        1905          1901      1909 
Year 

FIG.    254. — Prime    movers    used    in 
the  manufacturing  industries.* 


for  a  certain  installation  which  will  produce  the  energy  in 
that  installation  at  the  least  cost  per  kilowatt-hour.  There 
are  many  elements  which  must  be  considered  in  such  a  de- 
termination of  least  cost.  Some  of  these  will  be  described 
briefly  in  the  articles  which  follow.  Figs.  253  and  254  show 
respectively  the  horse-power  outputs  of  the  prime  movers 
used  in  central  stations  and  industrial  plants  in  the  United 
States 

332.  The  Application  of  Steam  Prime  Movers  should,  in  the 
case  of  very  small  installations,  be  restricted  to  locations  where 

•  E.  A.  Lof,   Coal  Age,  Feb.  6,  1915. 

281 


282 


CENTRAL  STATIONS 


[ART.  332 


coal  is  very  cheap  and  where  water  power  is  not  available. 
In  medium-capacity  installations,  say  those  of  from  300  to 
5,000  kw.,  steam  plants  may  be  more  economical  than  in- 
ternal-combustion engine  plants  or  those  hydro- electric  plants 
which  require  considerable  capital  expenditure  for  develop- 
ment. In  large  plants,  where  units  of  capacities  of  5,000  kw. 
and  upward  may  be  utilized  effectively,  modern  steam  turbo- 


Pounds  of  Steam  per  Kw.  Hour 

<: 

earn  Economy  Tests 
Steam  Turbines 
/  Values  Baseaf  on  - 
earn  Pressure          20 

1 

^ 

--5 

000 

K» 

,  / 

03 

01 

•^ 

*^~* 

S1 

1  ! 

i 

Superheat                   200  "  F. 
Condenser  Pressure     1  In.  Absol 

ufe 

N 

$00 

">  K 

y.,  / 

535 

\ 

*\^ 

^^ 

—  —  ' 

r*-~' 

•— 

-'<f 

-<oo 

VYV 

JL 

n 

-  —  . 

.2 

i£J 

£_/< 

£ 

1914 

4,000                   «jpOO                 I?p00                  16,000                 20,000             24,000 

Load  in  Kilowatts 

FIG.  255. — Graphs  illustrating  the  development  in  the  economy 
of  turbo  generators.  * 

generator  energy  generation  will  ordinarily  prove  much  more 
economical  than  generation  with  an  internal-combustion  engine. 
It  will  also  prove  more  economical  than  hydro-electric  genera- 
tion and  transmission  unless  the  investment  required  to 
develop  the  hydro-electric  properties  is  unusually  small. 

NoTE.f — "The  progress  of  the  steam  turbine  (Fig.  255)  has  been  so 
great  that  it  has  practically  displaced  the  gas  engine.  As  the  cost  of 
the  gas-engine  unit  is  probably  seven  or  eight  times  as  great  as  that  of  the 
turbine,  the  gas  engine  has  been  practically  put  out  of  the  running  insofar 

•  Copyright  by  Samuel  Inaull. 

t  H.  G.  Stott.  REPORT  or  EFFICIENCY  TEST  ON  30,000  KW.  CROSS-COMPOUND  STEAM 
TURBINE,  read  before  the  1916  annual  meeting  of  the  A.  S.  M.  E. 


SEC.  15] 


ADAPTABILITY  OF  STEAM 


283 


as  large  power-plant  work  is  concerned."  "Another  interesting  side 
light  in  this  matter  is  the  value  of  the  turbine  as  the  general  prime  mover 
in  competition  with  anything  else  that  could  be  cited.  Fifteen  years 
ago  hydro-electric  power  developments  were  looked  on  as  a  choice  in- 
vestment worth  lots  of  money  with  almost  any  cost  of  development. 
Water  powers  were  developed  that  cost  $200,  $250  and  $300  per  kw. 


- 35.000  Kw.  NorthWesT  Station  1915 
FIG.  256. — Four  types  of  generating  apparatus  which  have  been  use  by 
the  Commonwealth  Edison  Company  of  Chicago.* 

Today  you  could  not  get  money  for  an  investment  of  that  kind.  The 
steam  turbine  has  risen  so  high  in  efficiency  and  economy  and  decreased 
so  much  in  first  cost  that  it  has  driven  out  all  possibility  of  developing 
many  of  these  water  powers.  When  you  allow  for  the  fixed  charges,  the 
steam  turbine  can  make  power  more  cheaply  than  the  high-priced  hydro- 
electric development."  "  Consider  the  case  of  Niagara  Falls  where  there 
are  no  dams  required  and  where  there  is  practically  an  unlimited  supply  of 
power.  The  steam-turbine  plant  can  compete  with  Niagara  power  today 
as  long  as  the  load  factor  of  the  Falls  power  is  less  than  50  per  cent. 

•  Copyright  by  Samuel  Inaull. 


284 


CENTRAL  STATIONS 


[ART.  333 


The  only  chance  for  a  financially  successful  water-power  development  is 
on  the  basis  of  a  high  load  factor." 

333.  The  Advantages  of  Large  Turbo-generators  are  these: 
It  is  possible  to  obtain  as  much  as  six  times  the  output  capacity 
in  the  same  floor  space,  Fig.  256.     The  first  cost  per  kilowatt 
with  the  large  turbo  units  is  something  like  a  fifth  of  the  cost 
of  equivalent  reciprocating  engine  units.     Furthermore,  the 
water  rates,  that  is,  the  steam  consumptions  of  the  large 
turbines,  are  in  the  neighborhood  of  one-half  of  the  equivalent 
consumptions  of  reciprocating  engine  units. 

334.  Internal-combustion  Engine  Prime  Movers  Are  Effec- 
tively Used  at  the  present  state  of  the  art  only  in  plants  of 
small  and  medium  capacity  unless  the  cost  of  coal  or  of  an 


''-last  in  Ash  2% 
FIG.  257. — Losses  in  a  non-condensing  steam  plant.* 


f&mtionml 

Friction  XJ% 


hydro-electric  development  is  unusually  large.  In  small 
plants,  say  those  up  to  300  kw.  in  capacity,  the  internal- 
combustion  engine  prime  movers  may,  unless  the  cost  of  coal 
is  very  low,  be  more  economical  than  steam-engine  plants. 
This  is  particularly  true  of  small  plants  which  operate  only  a 
portion  of  the  24  hr.  For  a  small  central-station  plant  which 
operates  only  at  night  or  for  a  small  industrial  plant  which 
operates  only  during  the  day,  an  oil  or  gas  engine  usually 
would  generate  power  at  minimum  cost  because  such  an 
engine  does  not  involve  the  standby  losses  which  obtain  with 
the  steam  prune  movers.  As  suggested  in  the  above  note, 
where  a  large  power  output  is  required,  the  internal-combus- 
tion engine  has  been  put  entirely  out  of  the  running  by  the 

*  R.  H.  Fernald,  "  PRODUCES  GAS  FBOM  LOW-GRADE  FUELS,"  Practical  Engineer, 
Dec.  15,  1914,  p.  1200. 


SEC.  15] 


ADAPTABILITY  OF  STEAM 


285 


condensing  steam  turbine.  If  the  cost  of  development  is 
not  excessive  it  may  be  possible  to  generate  power  with  a 
waterwheel — hydraulic  turbine — cheaper  than  with  an  in- 
ternal-combustion engine.  The  producer-gas  plants,  which  are 
much  more  economical  than  an  ordinary  non-condensing 
medium-capacity  steam-engine  plant  (Figs.  257  and  258), 
cannot  usually  compete  with  the  condensing  turbo-generator 
outfit  unless  the  cost  of  fuel  which  may  be  used  in  the  producer 
is  very  low. 


Lost  m-> 
Exhaust 
23.17. 


FIG.  258. — Loss  in  a  suction  gas-producer  plant.* 

335.  Hydraulic  Prime  Movers  Are  Most  Economical  where 
the  cost  of  developing  the  hydro-electric  plant  is  not  excessive. 
the  cost  of  developing  the  hydro-electric  plant  is  not  excessive. 
It  appears  to  be  the  common  impression  that  merely  because 
the  water,  which  drives  the  turbines  or  waterwheel  in  a  hydro- 
electric plant,  costs  nothing  that  the  cost  of  the  power  de- 
veloped should  be  correspondingly  low.  This  is  far  from  the 
truth,  because  in  determining  the  total  cost  of  the  power 
developed  it  is  necessary  to  include  the  fixed  charges  (interest, 
depreciation,  insurance,  taxes,  and  the  like)  on  the  investment 
required  to  develop  the  hydro-electric  property.  If  it  is 
necessary  to  pay  these  fixed  charges  on  an  expensive  dam 
and  on  a  large  real-estate  investment  required  for  tne  storage- 
water  area,  and  then  in  addition  pay  the  fixed  charges  on  a 
long,  expensive  transmission  line,  such  fixed  charges  may 

*  R.  H  FERNALD,  "  PBODCCBB  GAS  mo*  LOW-OBADB  FCELA."  Practical  Engineer, 
Dec.  15,  1914.  p.  1200. 


286  CENTRAL  STATIONS  [ART.  336 

more  than  overbalance  the  cost  of  the  coal  and  attendance 
that  would  be  required  by  a  modern  steam  station.  Ihe 
fixed  charges  on  the  transmission  line  should,  usually,  properly 
be  included  with  those  on  the  hydro-electric  plant  and  develop- 
ment. Without  the  transmission  line  the  plant  would  be 
useless. 

336.  The  Development  of  Low-head  Hydro-electric  Plants 
is  likely  to  involve  excessive  cost.  On  the  other  hand,  if, 
due  to  local  conditions,  it  is  possible  to  install  a  hydro-electric 
plant  without  incurring  a  large  investment  in  dams,  storage 
reservoirs  and  transmission  lines,  hydro-electric  power  may  be 
developed  very  cheaply.  Some  of  the  high-head  plants  used 
in  the  West  involve  a  relatively  low  first  cost  per  kilowatt  of 
capacity  and,  hence,  can  produce  power  very  cheaply.  Some 
hydro-electric  plants  for  towns  and  factories  are  often  eco- 
nomical where  the  plant  can  be  located  close  to  he  load — so 
that  a  transmission-line  investment  is  not  required — par- 
ticularly if  an  expensive  dam  and  storage  reservoir  is  not 
necessary. 


SECTION  16 


STEAM    ELECTRICAL-ENERGY   GENERATING 
STATIONS 

337.  Steam  Plants  May  Be  Conveniently  Divided  Into  Four 
Classes.* — While  there  can  be  no  definite  dividing  line,  it  is 
convenient  to  consider  steam  plants  under  four  headings: 

(1)  small  plants,  to  include  all  of  those  under  300  kw.  capacity; 

(2)  medium  plants,  to  include  those  between  3,000  and  5,000 
kw.  capacity;  (3)  large  plants,  those  between  5,000  and  50,000 
kw.  capacity;  and  (4)  extra  large  plants,  which  include  those 
above  50,000  kw.  capacity. 


N01 

t-.- 

Graphs 
tire  Basea 
Pressure 
no  Super 

Has.  l,  1, 
onUSL 
28"Vaci 
heat. 
Nos.5ar 
ISO  U>.  I 
and  26" 

J,«rtd4 
b.  bage 
lurnand 

d6  art 
«,ge 
Sacuum. 

\,.-3<X> 
v    Non 

<<>Y.  Comp 
-Release 

urxt 

j£ 

pv 

KM.  Turt 

Kw.  Com 

•liss  Engi 

Based  or 
Pressure 

7# 

\ 

v\ 

\ 

-•aoo  KK 

rO 

Turbine 

500 
16 

•>v.  Turbir 

«-' 

,*\ 
'•ISO  Kw 

Turbine 

fa 

• 

...  800          1000         1700 

Rating  in  Kilowatts 

Fio.  259. — Graphs  showing  steam  consumptions  of  relatively-small  steam 
prime  movers  operating  condensing. 

338.  In  Small  Steam  Plants  it  is  usually  considered  good 
practice  to  install  only  one  or  two  units.  Often  only  one  unit 
of  sufficient  capacity  to  carry  the  entire  load  is  used.  The 
graph  of  Fig.  259  indicates  the  water  rates  of  some  relatively 

•  J.  W.  Shuster,  TENDENCIES  IN  CENTRAL  STATION  PRACTICE,  Electrical  Review, 
Mar.  3,  1917. 

287 


288 


CENTRAL  STATIONS 


[ART.  339 


small  steam  prime  movers.  It  will  be  noted  that  the  efficiency 
at  full-load  decreases  rapidly  as  the  size  of  the  turbine  de- 
creases. It  should  also  be  noted  that,  with  all  of  the  units,  the 
efficiency  decreases  rapidly  as  the  load  on  the  unit  decreases. 
Hence,  it  is  desirable  to  use  the  largest  units  possible  and  to 
always  operate  them  at  as  nearly  full-load  as  is  feasible.  Fre- 


FIG.  260. — Lay-out  for  a  small  belted  plant  driven  by  a  Corliss  and  a  high- 
speed engine. 

quently  it  is  desirable,  in  these  small  plants,  to  operate  non- 
condensing  because  the  fixed  and  operating  charges  on  the 
condensing  equipment  may  be  such  that  its  first  cost  is  not 
justified.  In  such  installations  where  simplicity  and  reliability 
of  service  are  the  most  important  factors,  a  high-speed  engine 
of  the  slide  valve  or  non-releasing  Corliss  valve  type  may  be 
used.  Or  instead  Corliss  engines  may  be  installed.  Non- 
condensing  reciprocating  steam  engines  are  more  economical 
than  non-condensing  turbines. 

339.  In  Considering  Belted  vs.  Direct-connected  Steam- 
engine  Units  for  Small  Plants  it  should  be  recognized  that  the 
cost  of  the  belted  outfit  is  always  lower  than  that  of 


SEC.  16] 


GENERATING  STATIONS 


289 


connected,  because  where  the  generator  is  belted  it  may  oper- 
ate at  high  speed — which  involves  low  generator  cost.  If  the 
generator  is  direct-connected  to  the  engine  its  speed  must  be 
the  same  as  that  of  the  engine,  which  is  relatively  low.  Fig. 


•-  .  ••  v  #> 

•  i  • 


FIG.  261. — Direct-connected    generating    unit.     A    high-speed    piston-valve 
engine  driving  a  small  alternator. 

260  shows  a  typical  belted  installation.  *  A  high-speed  engine, 
Ei,  is  direct-connected  to  the  jack  shaft  from  which  the  gen- 
erators are  belted.  A  Corliss  engine  is  also  belted  to  the  jack 
shaft.  Both  of  the  engines  are  used  to  pull  the  station  at 

•F.  W.  Salmon,  Practical  Enginttr,  Jun«  18,  1915. 
19 


290 


CENTRAL  STATIONS 


[Am.  340 


times  of  peak  load.  At  other  times  either  the  Corliss  engine, 
EZ,  or  the  high-speed  engine,  E\,  were  used  to  pull  the  load, 
depending  upon  which  could  operate  most  economically. 
Small  stations  of  this  general  character  may  give  satisfactory 
service  in  towns  where  the  cost  of  coal  is  low  and  where  relia- 
bility and  ease  of  operation  are  of  more  importance  than  high 
economy. 

340.  A  Small  Direct-connected  Generating  Unit,  similar  to 
that  of  Fig.  261,  may  be  used  where  economy  of  space  is  an 
important  factor.  A  unit  of  this  type  which  is  equipped  with 
a  small  slide  valve  engine  will  not  be  as  economical  as  one  hav- 
ing an  engine  with  an  automatic  cut-off  valve  gear. 

341.  Average  Full-load  Steam  Consumptions  of  Reciprocating  Steam 
Engines  of  Different  Types* 


Type  of  engine 

Range 
of  h.p. 

Speed 
in 
r.p.m. 

Steam 
pressure 
gage,   Ib. 
per  sq. 
in. 

Steam 
consump- 
tion per 
i.h.p.hr. 

High-speed 

Simple 
non-condensing 

30-150 

375-275 

80-110 

35-32 

Compound 
non-condensing 

100-150 

325-250 

100-150 

27-25 

Compound 
condensing 

100-300 

325-250 

100-150 

20-19 

Medium-speed, 
four-valve, 
non-releasing  gear 

Simple 
non-con  densing 

75-175 

225-200 

100-125 

29-26 

Compound 
non-condensing 

100-300 

210-180 

110-150 

23-21 

Compound 
condensing 

100-300 

210-180 

110-150 

18-16 

Slow-speed, 
"Corliss"  or  other  four- 
valve  with  releasing  gear 

Simple 
non-condensing 

100-350 

100-70 

80-125 

28-25 

Compound 
non-condensing 

200  and  up 

100-65 

110-175 

22-20 

Compound 
condensing 

250  and  up 

100-65 

110-175 

16-14 

*  George  Shaad  in  1910  STANDARD  HANDBOOK,  p.  676. 


SBC.  16] 


GENERATING  STATIONS 


291 


342.  A  Small  Alternating-current  Generator  Belted  to  a 
High-speed  Slide-valve  Engine  is  shown  in  Fig.  262.  Units 
of  this  character  may  be  used  in  small  stations  where  the  cost 
of  fuel  is  low  and  where  an  easily  handled  outfit  of  very  low 
first  cost  is  imperative. 


y?^w\\\w^\\w\\\^^ 

FIG.  262. — A  high-speed  simple  engine  belted  to  a  small  three-phase  generator. 

343.  Uniflow  Engines,  which  have  been  used  in  Europe  for 
a  considerable  period  but  which  have  only  recently  been  thor- 


STeam 

At/mission 

Valves. 


Fia.  263. — Sectional  elevation  through  the  cylinder  of  a  Skinner-pop- 
pet-valve uniflow  engine.  (Full  opening  of  exhaust  through  central 
ports.) 

oughly  developed  in  the  United  States,  afford  economical 
prime  movers  for  small  units.  A  uniflow  engine  is  one  in 
which  the  steam  (Fig.  263)  leaves  the  cylinder  at  its  middle 
point,  E.  The  steam  enters  the  cylinder  through  valves,  V, 
at  either  end  and  is  discharged  through  ports  in  the  middle 


292 


CENTRAL  STATIONS 


[ART.  343 


FIG.  264. — An  18"  X  24'  ,  200-r.p.m.,  Ames-Stumpf  Uniflow  engine  oper- 
ating condensing  at  141  Ib.  pressure,  superheat  approx.  100  deg.,  driving 
an  Allis-Chalmers  2500-kva.,  220-volt,  3-phase,  60-cycle,  generator  served  by 
a  125-volt  exciter.  (Ames.  Iron  Works,  Oswego,  N.  Y.)  This  is  installed  in 
the  plant  of  the  Winsor  (Vermont)  Electric  Light  Company. 


Flywheel  — - 


Exhaust  Pipe'' 

FIG.  265  — Sectional  Elevation  showing  construction  of  the  Ames-Stumph 
uniflow  engine. 


SEC.  16] 


GENERATING  STATIONS 


293 


which  are  opened  and  closed  by  the  cylinder  piston,  P,  which 
acts  as  an  exhaust  valve.  The  economical  performance  of 
the  uniflow  engine  is  due  to  the  fact  that  cylinder  heads  are 
always  maintained  at  the  same  temperature  so  that  there  is 
little  condensation  in  the  cylinder.  The  steam  always  flows 
in  one  direction.  With  the  simple  engine,  each  cylinder  head 


!- 

i 

g2S 

«/> 
•8" 

10 


tl 


-  Steam  Turbine,  300  KH.  3600R.PM,Conaensingf«rt  28- 

-  Vacuum  ISO  Lb.  per  Sq.  In  (Saturated)  Steam.  - 

-  Generator.  315  Kva.  at  80%  Power  Factor.  2300  Volts. 
_  3Pnast.  60  Cycles,  with  Direct  Connected  5  Kw.  Exciter. 
-.  UniflowEngtne.  21  \22*,200RRU,12SLb.per  Sq.ln     • 

-  Steam  Pressure  f  Saturated.) 

-  Generator,  2SO  Ktv,  312  K*a.  00%  PontrFactor.  2300 
Wts,  3 Phase.  200 k P.M. 


0  50  WO          150          TOO         250          TOO 

Kilowatts  Load  nt  60  1.  Pow«r  Factor 

Fio.  266. — Characteristic  performance  graphs  of  the  uniflow  engine.* 

alternately  is  heated  and  cooled  as  the  steam  is  permitted 
to  enter  and  exhaust  from  the  cylinder.  Uniflow  engines  can 
be  used  with  superheated  steam,  which  results  in  further 
economies.  It  appears  that  modern  uniflow  engines  in  capaci- 
ties up  to  about  500  h.p.  are  as  economical  as  any  of  the  other 
types  of  steam  prime  movers.  They  can  be  obtained  for  both 
condensing  or  non-condensing  service  or  for  combined  con- 
densing and  non-condensing  operation.  Fig.  264  shows  a  uni- 
flow engine  direct-connected  to  an  alternating-current  gen- 
erator which  is  served  by  a  belted  exciter.  Fig.  265  shows 

•E    Hagenlocher,  "CHARACTERISTICS  or  UNIFLOW  GENERATING  UNITS,"  EUctrical 
World,  Feb.  10,  1917,  p.  260. 


294 


CENTRAL  STATIONS 


[ART.  343 


the  construction  of  an  engine  of  the  type  reproduced  in  Fig. 
264.  Tests  of  uniflow  engines  operating  non-condensing  at 
140  Ib.  boiler  pressure  have  shown  an  economy  of  30  Ib.  of 


load 


FIG.  266A. — Graphs  showing  water  rates  01  a  Skinner  21  in.  X  22  in. 
uniflow  engine  running  non-condensing  and  condensing,  saturated  steam  at 
140  Ib.  (This  and  the  next  illustration  are  from  data  on  engines  which  were 
direct-connected  to  generators  of  200  true  kw.  capacity  and  would  be  con- 
sidered 320  h.p.  engines  at  normal  full  load.  However,  the  engines  are  good 
for  400  indicated  h.p.  for  maximum  continuous  operation.) 


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0 

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Indicated  Horsepower 

FIG.  266B. — Graph  showing  water  rate  of  a  21  in.  X  22  in.  Skinner  uniflow 
engine  operating  at  a  pressure  of  149.1  Ib.  With  a  superheat  of  102.5 
deg.  F. 

steam  per  kw.-hr.  when  operating  at  full-load.  Fig.  266  gives 
typical  economy  graphs  for  a  uniflow  unit  as  compared  with 
a  turbo-generator.  It  will  be  noted  that  the  performance 


SEC.  16] 


GENERATING  STATIONS 


295 


graph  of  the  uniflow  engine  near  the  full-load  point  is  almost 
horizontal.  That  is,  its  efficiency  does  not  change  greatly  with 
change  in  load,  around  the  full-load  point.  This  feature  is 
shown  in  the  performance  graphs  of  Figs.  266 A  and  266 B 


FIG.  267. — Sectional  elevation  through  the  engine  room  at  the  Springfield 
(111.)  Capitol. 


FIG.  2G8. — Plan  of  the  uniflow  engine  plant  and  the  boiler  plant  in  the  Spring- 
field (111.)  Capitol. 

344.  A  Uniflow  Engine  Installation  is  shown  in  Figs.  267 
and  268.  These  illustrate  the  equipment  used  for  supplying 
electrical  energy  to  the  State  buildings  at  Springfield,  Illinois. 
Three  direct-connected  engine-driven  units  are  installed. 
Each  unit  comprises  a  Chuse-Poppet  valve  engine  direct-con- 
nected to  a  Westinghouse  direct-current  generator.  One  18 


296  CENTRAL  STATIONS  [ART.  345 

by  21-in.  engine  drives  a  200-r.p.m.  150-kw.  125-volt  gener- 
ator. Each  of  the  other  units  is  driven  by  a  22  by  28-in.  en- 
gine, the  generators  being  rated  at  300  kw.  and  operated  at  150 
r.p.m.  The  150-kw.  unit  is  operated  when  the  load  is  light — 
on  Sundays  and  at  night.  The  present  day  load  is  handled  by 
one  of  the  300-kw.  units. 

345.  Steam-turbine  Stations  have  recently  attained  a  posi- 
tion of  great  importance.     This  is  particularly  true  of  the 
stations  wherein  units  of  20,000  kva.  and  greater  capacities 
may  be    utilized.       As  suggested  above,   a  modern,   large, 
steam  turbine  generates  power,  under  ordinary  conditions, 
more  economically  than  can  a  prime  mover  of  any  of  the  other 
types.     Steam  turbines  are  now  widely  used  even  in  capacities 
of  under  500  kw.     They  are  also,  because  of  their  simplicity 
and  reliability,  used  for  driving  the  auxiliary  apparatus  in  a 
station  such  as  condenser  pumps  and  feed  pumps.     High  steam 
pressures  are  desirable,  hence  pressures  ranging  from  150  to 
250  Ib.  are  encountered.     Since  superheating  also  increases 
the  economy,  the  steam  is  usually  superheated  from  125  to 
150  deg.     High  vacuums  ranging  from  28  to  29  in.  are  em- 
ployed. 

EXAMPLE.* — A  500-kw.  turbo  unit,  when  operating  on  a  steam  pressure 
of  150  Ib.  and  125  deg.  of  superheat  and  a  28-in.  vacuum  has  a  steam  con- 
sumption of  approximately  17%  Ib.  per  kw.-hr.  A  15,000-kw.  unit  op- 
erating at  a  steam  pressure  of  250  Ib.  with  125  deg.  of  superheat  and  a 
29-in.  vacuum  has  a  steam  consumption  of  only  11%  lb-  per  kw.-hr. 

346.  Why  the  Large  Steam  Turbine  Is  so  Economical  may  be 
explained  thus:  First,  since  no  lubrication  is  necessary  in  the 
parts  of  the  turbine  with  which  the  steam  comes  into  contact, 
a  higher  degree  of  superheat  is  possible  with  it  than  with  the 
reciprocating  engine,  for  which  cylinder  lubrication  is  neces- 
sary.    Second,  when  the  turbine  is  used  condensing,  as  it  must 
be  for  maximum  economy,  the  turbine  utilizes  the  expansive 
power  of  the  steam  down  to  the  highest  vacuum  which  can 
be  developed.     The  reciprocating  engine  can  utilize  the  expan- 
sive power  of  the  steam  only  down  to  possibly  a  25-  or  26-in. 
vacuum.     If  an  endeavor  is  made  with  a  reciprocating  engine 

•  E.  A.  Lof. 


SEC.  16] 


GENERATING  STATIONS 


297 


to  expand  the  pressure  of  a  high  vacuum,  the  low-pressure 
cylinders  and  the  reciprocating  parts  associated  therewith 


90  100  110 

Load  7.  Rowing 

FIG.  269. — Water  rate  graphs  of  some  medium-capacity  condensing  turbines.* 

would  be  so  large  that  their  cost  and  frictional  losses  would 
more  than  offset  the  theoretical  economies  resulting  therefrom. 
It  is  evident,  therefore,  from 
the  foregoing,  that  a  much 
greater  part  of  the  total 
energy  in  the  system  can  be 
realized  with  the  turbine  than 
with  the  reciprocating  engine. 
Another  factor  which  renders 
the  turbine  so  economical  is 
its  low  first  costs  per  unit  of 
power  output.  This  is  be- 
cause the  turbine  is  in- 
herently a  high-speed  prime 
mover. 

347.  The  Efficiency  Graph 
of  the  Turbine  is  almost  flat 
Fig.  269.  It  follows  that  a 


I  98        190?         1906         1910 
1900         1904         1908         1912 
Years 


FIG.  270  — Graphs  showing  how 
the  capacities  of  steam-turbo-gen- 
erator  units  have  increased  with  the 
development  of  the  art. 


turbine  operates  at  good  econ- 
omy over  a  wide  range  of 
loads. 

348.  The    Growth    in    the 

Capacities  of  Turbo-generator  Units  has  been  almost 
phenomenal.  As  suggested  in  the  graph  of  Fig.  270, 
single  units  have  been  manufactured  having  outputs  of 

*  Reginald  J.  S.  Pigott,  STANDARD  HANDBOOK,  July,  1915 


298  CENTRAL  STATIONS  [ART.  349 

60,000  kw.  The  manufacturers  assert  that  there  is  no 
reason  why  units  of  greater  capacities  than  60,000  kw. 
cannot  be  built  provided  there  is  a  demand  for  them.  Fig. 
270  indicates  the  gradual  increase  in  the  capacities  of  the  gen- 
erating units  in  the  stations  of  one  central-station  company. 
349.  A  Comparison  of  the  Steam  Turbine  and  the  Steam 
Engine  as  regards  their  applications  is  summarized  in  the 
following  data*  which  relate  particularly  to  units  of  less  than 
500  h.p. 

NOTE. — Applicability  of  Turbines. — 1.  Direct-connected  units,  oper- 
ating condensing.  60-cycle  generators  in  all  sizes,  also  25-cycle  gener- 
ators above  1,000  kw.  capacity.  Direct-current  generators  in  sizes  up 
to  1,000  kw.  capacity,  including  exciter  units  of  all  sizes.  Centrifugal 
pumping  machinery  operating  under  substantially  constant-head-and- 
quantity  conditions  and  moderately  high  head,  say  from  100  ft.  up,  de- 
pending upon  the  size  of  the  unit. 

Fans  and  blowers  for  delivering  air  at  pressures  from  1^  in.  water 
column  to  30  Ib.  per  sq.  in. 

2.  Direct-connected  units,  operating  non-condensing  for  all  the  above 
purposes,  in  those  cases  wherein  steam  economy  is  not  the  prime  factor 
or  where  the  exhaust  steam  can  be  completely  utilized,  and,  in  the  latter 
case,  particularly  where  oil-free  exhaust  steam  is  desirable  or  essential. 

3.  Geared  units,  operating  straight  condensing  or  non-condensing  for 
all  the  above-mentioned  applications,  and  in  addition,  many  others  which 
would  otherwise  fall  in  the  category  of  the  steam  engine,  on  account  of 
the  relatively  slow  speed  of  the  apparatus  to  be  driven. 

Applicability  of  Engines. — 1.  Non-condensing  units,  direct-connected 
or  belted  and  used  for  driving: 

(a)  Electric  generators  of  all  classes  excepting  exciter  sets  of  small 
capacity,  unless  belted  from  the  main  engine. 

(6)  Centrifugal  pumping  machinery,  operating  under  variable  head 
and  quality  conditions  and  at  relatively  low  head,  say  up  to  100  ft., 
depending  on  the  capacity. 

(c)  Pumps  and  compressors  for  delivering  water  or  gases  in  relatively 
small  quantities  and  at  relatively  high  pressures — in  the  case  of  pumps 
at  pressures  above  100  Ib.  per  sq.  in.  and  in  the  case  of  compressors  at 
pressures  from  1  Ib.  per  sq.  in.  and  above. 

(d)  Fans  and  blowers  (including  induced  draft  fans)  for  handling  air 
in  variable  quantities  and  at  relatively  low  pressures,  say  not  over  5-in. 
water  column. 

*  J.  S.  Barstow,  TURBINES  vs.  ENGINES  IN  UNITS  OF  SMALL  CAPACITIES,  a  paper  read 
before  the  A.  S.  M.  E. 


SEC.  16] 


GENERATING  STATIONS 


299 


(e)  Line  shafts  of  mills,  where  the  driven  apparatus  is  closely  grouped 
and  the  load  factor  is  good. 

(/)  All  apparatus  requiring  reversal  in  direction  of  rotation,  as  in 
hoisting  engines  and  engines  for  traction  purposes. 


4  6          12          16         70 

Rating  in  100  Kw. 

Fio.   271. — Steam    consumptions  of 
small  turbines. 


i-~F 


sfew 


734S61&9IO 
Rating  in  1000  Kw. 

Fio.  272. — Steam  consump- 
tions of  steam  turbines  of 
moderate  capacities. 


460 

Maximum  Rating  in  1000  Kw. 

Fio.  273. — Approximate  relative  costs  of  turbo-generator  units.  (Cost 
of  labor  and  materials  are  now  fluctuating  so  widely  that  it  is  not  feasible  to 
give  actual  costs.) 


2.  Condensing  units  direct-connected  or  belted,  for  all  the  above  pur- 
poses, particularly  where  the  condensing  water  supply  is  limited,  and 
where  the  water  must  be  recooled  and  recirculated. 


300 


CENTRAL  STATIONS 


[ART.  350 


350.  The  Steam  Consumptions  of  Small  and  Medium- 
Capacity  Turbines*  are  indicated  by  the  graphs  of  Figs.  271 
and  272.  The  graph  of  Fig.  271  indicates  the  economics 
which  may  be  effected  through  the  use  of  a  high  steam  pres- 
sure and  superheat.  Fig.  273  indicates  the  approximate  cost 
of  small  and  moderate  capacity  turbo-generator  units. 


3  Phase,  60  Cycle 
.-''Generators 


Low- Pressure  Turbines-, 


FIQ.  274. — Three-cylinder,  two-stage  turbo-generator  unit.     Capacity  is 
60,000  kw. 

351.  The  Cross-compound  Principle  As  Used  in  Large 
Turbines  is  shown  in  Fig.  274  which  illustrates  the  60,000- 
kw.  three- cylinder,  two-stage  unit  purchased  by  the  Inter- 
borough  Rapid  Transit  Company  of  New  York.  Tests  on 
this  unit  indicate  at  the  point  of  maximum  efficiency  a  water 
rate  of  11.25  Ib,  per  kw.-hr. 

•  David  Elwell  in  a  paper  presented  before  a  New  England  National  Electric  Light 
Association. 


SEC.  16] 


GENERATING  STATIONS 


301 


352.  An  Example  of  a  Medium-capacity  Turbo-generator 

Station  is  shown  in  Figs.  275  and  276,  which  illustrate  a 
plant  operated  by  the  Arkansas  Light  and  Power  Company.* 


FIG.  275. — The  boiler  room  in  which  the  wood  waste  is  burned. 

In  this  installation  wood  waste  from  a  saw  mill  and  a  planing 
mill  is  used  for  firing  the  boilers  B\  to  £8-  The  boiler  house 
(Fig.  275)  is  located  some  distance  away  from  the  generating 


FIG.  276. — Turbo  generator  station  utilizing  steam  generated  with  saw  and 
planing  mill  waste. 

station  (Fig.  276),  the  boiler  house  being  operated  by  a  com- 
pany which  owns  the  saw  and  planing  mills.  This  company 
sells  steam  to  the  Light  and  Power  Company  for  the  operation 
of  the  turbines  G\  and  Gi. 

•  J.  B.  Woods.  Eltetrieal  World,  Apr.  14,  1917. 


302 


CENTRAL  STATIONS 


[ART.  353 


353.  A  Design  for  a  Turbo  Plant  Having  Only  One  Generat- 
ing Unit  is  shown  in  Fig.  277.     This  plant  is  for  the  operation 
of  a  flour  mill.     A  small  9  by  12  gas  engine,  E,  installed  in 
the  engine  room,  is  used  for  pulling  the  load  at  nights  and  at 
other  times  when  it  would  be  uneconomical  to  operate  the 
turbine,  T,  at  the  small  loads  then  existing. 

354.  An  Example  of  a  Large  Turbo -generator  Installation 
is  shown  in  Figs.  278  and  279  which  shows  the  Essex  Station 
of  the  Public  Service  Corporation  of  New  Jersey.     This  is 
said  to  be  one  of  the  most  efficient  steam  stations  ever  con- 


Four-Stage  6.C.  Impulse      ISO  Ff.  Stack 
/Turbine,  3,600  R.RM.     ;'6  Ft.  Dia. 


JoColeAut. 

;'antl  Troy  Engine 


9"xl2"-'  '-60  Km,  480  K,  J 

Gas  Engine  Phase  Generator 

FIG.  277. — Plan  view  of  the  plant  of  the  Commercial  Milling  Company.* 

structed.  The  equipment  comprises  eight  1,373-h.p.  boilers 
and  the  necessary  superheaters  and  boiler-room  auxiliaries. 
The  generating  equipment  consists  of  two  25,000-kva.  13,200- 
volt  General  Electric  Company  turbo-generator  units,  T,  in 
Fig.  278. 

355.  A  Moderate-capacity  Turbo-generator  Plant  is  shown 
in  Fig.  280,  which  illustrates  the  construction  and  general 
arrangement  of  the  power  plant  of  the  Remington  Arms  and 
Ammunition  Company  at  Bridgeport,  Conn.     The  generating 
unit  shown  has  a  capacity  of  2,000  kw. 

356.  Boilers  for  Steam-generating  Stations  are,  in  moderate 

*  Power,  Dec.  22,  1914,  p.  870. 


SEC.  16] 


GENERATING  STATIONS 


and  large-capacity  plants,  always  of  the  water-tube  type, 
inasmuch  as  this  is  the  only  type  which  will  permit  of  the  use 
of  high  steam  pressures  and  the  ready  utilization  of  economizers 
and  superheaters.  While  in  some  quarters  the  feeling  exists 


Circulating.-' 

Pump  Circulating  Pump 

Suction 

FIG.  278. — Sectional  elevation  of  the  turbine  building  of  the  Essex  Station. 

that  1,200  h.p.  is  as  great  a  capacity  as  is  desirable  for  one 
unit,  boilers  rated  at  2,400  h.p.  each  have  been  used  in  certain 
installations.  In  medium-capacity  plants,  the  capacity  of 
each  boiler  unit  is  about  500  h.p.  Fire-tube,  that  is  return- 
tubular,  boilers  are  now  used  only  in  the  small  non-condensing 
plants. 


304 


CENTRAL  STATIONS 


[ART.  356 


Turbine 

Room  \   Future     \ 


r.e'H!gh- 

'  Pressure  Line 


'Switch  \      ''Future          "C°-n^/f  Property  Line--' 

House     \oadw0y 

FIG.  279.— Genera]  lay-out  of  the  Essex  Station  of  the  Public  Service  Cor- 
poration of  New  Jersey. 


Coal  Bunker 

ity^  Tons 
•per  Lineal  Foot 


Air Chamber-' '/ 
Ash  Pit — '' 


36  In.  Intake-' 

FIG.  280.— Boiler  house  and  generating  station  of  the  Remington  Arms  and 
Ammunition  Company,  Bridgeport,  Conn. 


SEC.  16]  GENERATING  STATIONS  305 

357.  In  Arranging  the  Boilers  any  one  of  several  methods 
may  be  followed.  The  rows  of  boilers  may  be  arranged  facing 
an  aisle  running  the  short  length  of  the  station  or  they  may 
be  laid  out  in  either  single  or  double  rows  along  a  fire  aisle. 
The  illustrations  show  some  of  the  different  methods  of  boiler 
arrangement  that  have  been  utilized  in  practice. 


SECTION  17 


INTERNAL-COMBUSTION-ENGINE  STATIONS 

358.  In  Internal-combustion  Engine  Stations  a  prime  mover 
may  be  either  a  gas  engine,  usually  supplied  with  producer  gas, 
or  an  oil  engine.     The  producer-gas  installations  are  suitable 
only  for  plants  of  medium  or  relatively  large  capacities  and, 
as  above  suggested,  a  steam  turbine  is  more  economical  in 
most  installations  of  this  character.     The  oil  engines  have  been 
used  to  a  considerable  extent  in  medium-capacity  plants  in 

locations  where  the  cost  of 
oil  is  low.  For  very  small 
plants  which  give  service 
for  a  portion  of  the  time, 
such  as  lighting  plants 
operating  in  small  towns 
only  during  the  night, 
small  oil  engines  are  more 
economical  than  prime 
movers  of  any  other  type. 
Large  gas  engines  have 
been  used  with  great  suc- 
cess in  steel  plants  where 
by-product  gas  for  every 
"hoVsT.6  r±r»rc9e  -rt-JSJ:  operation  is  obtained  from 

Westinghouse  vertical  gas  engine.)  the    blast    fumaCCS.      Such 

installations  are,  however, 

of  a  specific  character  and  can  hardly  be  classed  as  central- 
station  plants.  Gas  engines  have  been  built  for  use  in  these 
plants  in  capacities  of  4,000  to  6,000  h.p.  output  each. 

359.  The  Efficiency  of  an  Internal-combustion  Engine  In- 
creases With  the  Load  (Fig.  281),  so  that  the  most  efficient 
load  for  any  internal-combustion  engine  is  the  greatest  load 
which  that  engine  will  carry.     It  follows  that  internal-com- 
bustion engines  should  be,  and  are,  rated  on  the  maximum 
basis.     That  is,  they  are  not  rated  with  overload  capacities 


Brake  Horsepower 
FIG.    281. — Typical    internal-combus- 


EC.  17]      INTERNAL-COMBUSTION-ENGINE  STATIONS      307 


Vaporizer 


ria.  282. 


. — Section  through  a  small  low-pressure  oil  engine  of   the  "hot- 
bulb"  or  "hot-ball"  type. 


26  Ft.-  — 

Cooling-Water  Tank-'' 
5/7.,  9  Ft.  High 


Scale  in  Feet 
FIG.  283.  —  General  lay-out  of  small  oil-engine 


plant. 


308 


CENTRAL  STATIONS 


[ART.  360 


360.  Oil  Engines  for  Small-town  Electric-lighting  Plants 
are  usually  of  the  hot-bulb  (Fig.  282)  type.  These  can  be 
operated  on  crude  oil  or  kerosene.  It  has  been  found  most 
economical  in  certain  small  plants  to  use  kerosene  rather  than 


.-•Composition  Roofing 


FIG.  284. — Transverse  section  of  station. 

crude  oil  because,  in  order  to  obtain  a  low  rate  per  gallon,  the 
crude  oil  must  be  purchased  in  tank-car  quantities.  In  one 
of  these  small  plants  a  considerable  period  elapses  before  a 
tank  of  oil  can  be  consumed.  Hence,  the  charges  on  a  con- 

.••Composition  Roofing 


Corrugated  Iron  on- 
Timber  Forms 


FIG.  285. — Longitudinal  section  of  station. 

tainer  required  to  store  the  tank  of  oil,  more  than  offsets  the 
difference  in  cost  between  the  crude  oil  and  the  kerosene. 
Kerosene  can  be  obtained  in  practically  any  locality  in  rela- 
tively small  drums. 


SEC.  17]      INTERNAL-COMBUSTION-ENGINE  STATIONS     309 

361.  The  Lay-out  of  a  Small  Oil-engine  Plant  is  shown  in 
Fig.  283.  A  plant  of  this  type  might  serve  its  consumers 
through  a  distribution  system  similar  to  that  diagrammed  in 
Fig.  241.  The  prime  mover  consists  of  a  30-h.p  ,  400-r.p.m. 
Remington  oil  engine  which  drives  a  15-kva.  belted  generator. 
An  economical  construction  for  a  building  to  house  an  equip- 
ment of  this  character  is  shown  in  Figs.  284  and  285. 


SECTION  18 
HYDRO -ELECTRIC  STATIONS 

362.  Hydro-electric  Stations  May  Be  Divided  Into  Three 
General  Classes :  (a)  low-head  stations,  4  to  25  ft. ;  (6)  medium- 
head  stations,  25  to  300  ft.;  and  (c)  high-head  stations,  300  ft. 
to  3,000  ft.  and  up.     Fig.  286  gives  a  graphic  definition  of  the 
meaning  of  the  word  "head." 

363.  Waterwheels  May  Be  Divided  Into  Three  General 
Classes :  (a)  Gravity,  (&)  reaction,  and  (c)  impulse.     The  char- 
acteristics of  wheels  of  each  type  and  illustrations  thereof  will 
be  given  below. 

Headiest 
[InHfadRace 
Heart  Effective 


'*HeadLost!n  Tail  Ract 
FIG.  286. — Graphic  definition  of  the  term  "head." 

364.  A  Gravity  Wheel  (Figs.  287  and  287A)  is  one  which 
develops  its  power  by  virtue  of  the  weight  of  the  water  falling 
through  a   distance   equal  to  the  head.     The  falling  water 
carries  with  it  as  it  goes  down  the  buckets  which  catch  it  and 
thus  develops  power. 

365.  A  Reaction  Wheel  (Figs.  288  and  289)  is  one  which 
develops  its  power  by  virtue  of  the  reactive  pressure  of  the 
streams  of  water  upon  the  movable  buckets  from  which  the 
streams  are  forced  by  the  head  of  the  water  above. 

366.  An  Impulse  Wheel  (Fig.  290)  is  one  which  develops 
its  power  by  virtue  of  the  force  exerted  by  a  stream  of  water 
which  issues  from  a  nozzle  or  guide  and  impinges  on  buckets 

310 


SEC.  18] 


HYDRO-ELECTRIC  STATIONS 


311 


' Surface  of  Standing  Tall  Water 

FIG.  287. — Gravity-type  waterwheel,  the  most  efficient  wheel  for  very  low 
heads  and  small  quantities  of  water.  (Manufactured  by  the  Fitz  Water 
Wheel  Company,  Hanover,  Pa.) 


Gravity  Water 
.Wheel 


•  '/<;-\  ^ )  fc.;-         : 

, — A  combination  pumping  and  electric-light  plant  driven  by  e 
eravitv  waterwheel. 


FIG.  287 A.— A 


312 


CENTRAL  STATIONS 


[ART.  367 


on  the  rotating  wheel.     The  nozzles  or  guides  are  stationary 
and  the  wheel  rotates. 

NOTE  that  the  gravity  wheel  develops  its  power  solely  by  virtue  of 
the  weight  of  the  falling  water  and  that  the  reaction  and  impulse  wheels 
develop  their  power  by  virtue  of  the  potential  energy  due  to  the  weight 
of  the  water  which  is  first  changed  into  kinetic  energy. 


-  -GaTe-Regulating 
Hand  Wheel 


FIQ.  288. — Illustrating  the  principle  of  the  reaction  wheel.     (This  shows  a 
cylinder-gate,  horizontal-type  turbine  mounted  in  a  steel  flume.) 


FIQ.  289. — Runner  of   a   mixed- 
flow  turbine. 


FIQ.    290. — Impulse    wheel    and 
nozzle. 


367.  The  Efficiency  of  a  Waterwheel  decreases  both  above 
and  below  the  point  of  maximum  efficiency,  as  shown  in  Fig. 
291.  This  fact  must  be  recognized  in  selecting  the  generator 
which  is  to  be  driven  by  a  waterwheel  unit. 


SEC.  18] 


HYDRO-ELECTRIC  STATIONS 


313 


368.  The  Applications  for  the  Waterwheels  of  the  Three 
Different  Types  May,  in  a  general  way,  be  given  thus:  Gravity 
wheels  such  as  that  illustrated  in  Fig.  287  are  desirable  only  for 
very  low  heads  and  for  the  development  of  relatively  small 
power  outputs.  They  have  been  used  successfully  in  certain 
very  small  central-station  installations,  in  which  the  generator 
is  belted  to  the  wheel  shaft.  The  reaction  wheels  are  best 
adapted  to  relatively  low  heads  and  large  quantities  of  water. 
In  recent  years  reaction  wheels  have  been  used  for  all  ranges 


O-fe  0.7  08  09 

Speed  Factor 

FIG.  291. — Efficiency  graphs  of  a  48-in.  turbine.  (The  wheel  is  designed 
to  operate  at  a  speed  such  that  its  peripheral  velocity  is  75.8  per  cent,  of  the 
speed  or  velocity  of  the  spouting  water.  The  governor  associated  with  it 
holds  it  constant  at  this  speed.  But  it  is  obvious  from  the  above  that  when 
operating  at  partial  loads,  at  less  than  0.78  gate,  the  efficiency  decreases 
rapidly.) 


of  head  from  600  to  700  ft.  for  large  units.  The  impulse  wheel 
is  best  adapted  to  high  heads  and  small  quantities  of  water. 
Thus,  for  heads  greater  than  200  ft.  and  of  small  flow  of  water, 
the  impulse  wheel  is  the  most  effective  prime  mover.  The 
efficiencies  of  waterwheels  may,  for  large  units,  be  as  great  as 
80  to  90  per  cent. 

369.  The  Names  of  the  Elements  of  a  Hydro-electric 
Installation  are  given  graphically  in  Fig.  292.  Obviously  the 
typical  arrangement  shown  in  the  illustration  can  not  be 
followed  in  many  instances.  However,  the  nomenclature 
there  given  is  of  general  application. 


314 


CENTRAL  STATIONS 


[ART.  369 


FIG.  292. — Illustrating   the   nomenclature   of   the   essential   elements   of 
hydro-electric  development. 


_ 


FIG.  293. — Low-head,  vertical  water  wheel  generator  station.     (Electrical 
Machinery  Company.) 


SEC.  18]  HYDRO-ELECTRIC  STATIONS  315 

370.  The    General   Tendency    in   Hydro-electric    Station 
Design  has  been  thus  outlined  by  W.  R.  Thompson.*     There 
is  a  tendency  to  do  away  with  the  clear  spillway,  hence,  re- 
servoir-level-controlling devices,  either  in  the  form  of  Tainter 
gates  or  automatic  flashboards,  are  used.     Frequently,  the 
nearly  constant  reservoir  level  has  many  advantages  such  as 
minimizing  the  investment  in  reservoir  land  to  that  land  which 
is  ordinarily  submerged.     It  insures  the  advantage  of  more 
uniform  head  on  the  plant  and  facilitates  the  handling  of  the 
ice  which  forms  on  the  reservoir.     This  latter  consideration  is 
important  on  logging  streams  where  logs  are  imbedded  in  the 
ice.     The  modern  tendency,  especially  for  low-head  plants,  is 
toward  units  consisting  of  the  vertical,  single-runner,  large- 
capacity  turbine  (Fig.  293)  with  a  direct-connected  generator. 
Where  a  set  of  this  type  is  used,  economy  in  space  results,  and 
maximum  reliability  is  insured,  due  to  the  accessibility  of  the 
wearing   parts  for  inspection,   adjustment,   lubrication  and 
repairs.     The  enclosed  and  heated  forebay  is  now  being  used 
in  Northern  climates  to  prevent  the  interference  by  ice  with 
operation.     The  continued  use  of  a  submerged  forebay  is 
justified  because  it  eliminates  practically  all  floating  materials 
from  the  rack.     Where  conditions  affecting  the  choice  and 
size  of  units  permits,  there  is  a  tendency  to  reduce  the  number 
of  units  in  the  plant  to  say  three  or  four,  rather  than  to  have  a 
large  number  of  relatively  small  units. 

371.  The   Typical   Arrangement  of   a   Low-head  Hydro- 
electric Generating  Equipment  is  shown  in  Fig.  293.     This 
provides  about  as  simple  and  effective  arrangement  as  can  be 
designed.     The  turbine  and  the  generator  are  both  of  the 
vertical  type  and  are  direct-connected  so  that  there  is  no  un- 
necessary friction  lost  in  gearing  or  belting.     The  weight  of 
the  waterwheel,  the  pressure  of  the  downward  water  thrust 
and  the  weight  of  the  revolving  part  of  the  alternator  are 
carried  by  a  thrust  bearing  located  in  the  top  of  the  generator. 
The  guide  bearings  are  self-aligning  so  that  cramping  can  not 
occur.     No  step  bearing  is  necessary  or  provided  in  the  water- 
wheel. 

•  W.    R.    Thompson,  TENDENCY  IN  CENTRAL  STATION  DEBION.  Electrical    Review. 
Mar.  3.  1917. 


316 


CENTRAL  STATIONS 


[ART.  372 


372.  A  Low-head  Hydro-electric  Plant  with  a  Horizontal 
Turbine  is  shown  in  Fig.  294.  With  this  arrangement  the 
generator  is  direct-connected  to  the  turbine  shaft  and  the 
water  is  impounded  against  one  of  the  station  walls.  A 
modern  low-head  relatively  large-capacity  hydro-electric 


..-Heart  6at< 


FIG.  294. — A  low-head,  horizontal-turbo-generator-unit  installation. 
Leffel  &  Co.) 


(James 


station  is  illustrated  in  Fig.  295.  In  this  plant  ten  turbines 
each  developing  10,800  h.p.  on  a  30-ft.  head  and  operating 
at  53  r.p.m.  are  installed.  The  generators  are  6,600-volt, 
three-phase,  60-cycle  machines  and  are  rated  10,000  kva. 

373.  A  Typical  High-head  Hydro-electric  Station  is  shown 
in  Fig.  296.     The  impulse  wheel  is  of  the  Pelton  type.     The 


SEC.  18] 


HYDRO-ELECTRIC  STATIONS 


317 


governing  of  impulse  wheels  is  effected  by  deflecting  the  stream 
away  from  the  buckets  or  by  throttling  it.  Because  of  the 
high  heads  under  which  the  impulse  wheels  usually  operate, 
it  is  dangerous  to  attempt  to  govern  by  throttling  alone, 
hence  a  system  of  governing  which  combines  throttling  and 
stream  deflection  has  been  adopted. 


Blower  for 
Generator 
Ventilation 

<-!v 6-3*  C.  to  C.  Rails 

j< 26  rt.-,—-> 

••-— mrt.  Total  Wi'dlh  of  Power  Ho. 


-   32 Ft.  t}ln.- 
-35 Ft.  4 In.-- 


Fio.  295. — Hydro-electric   station  of   the   Cedar  Rapids  (Canada).  Manu- 
facturing and  Power  Company  (General  Electric  Review). 


374.  The  Largest  Hydraulic  Single-runner  Turbine  Ever 
Built  is  of  31,000-h.p.  capacity  (Fig.  297)  and  is  to  be  installed 
at  the  aluminum  plant  on  the  Yadkin  River  in  North  Carolina. 
The  head  is  188  ft.  The  speed  under  an  effective  head  of  188 
ft.  is  150  r.p.m.  An  efficiency  exceeding  91  per  cent,  at  full- 
load  is  expected.  The  generators  are  13,200-volt  machines 
rated  at  18,000  kva.  The  turbines  were  built  and  installed  by 


318 


CENTRAL  STATIONS 


[ART.  375 


the  Allis-Chalmers  Company  and  the  generators  and  exciters 
by  the  General  Electric.  As  shown  in  Fig.  298  the  plant  con- 
tains three  of  these  31,000  h.p.  units.  No  transformers  are 
required  in  the  station,  inasmuch  as  the  energy  is  generated 
at  13,200  volts  which  is  also  the  transmission  voltage.  Fig. 
299  gives  an  idea  of  the  complete  development. 


iSFtSln. 
JS.OOQVolts... 

46Ft.3ln.-'-»     "~T.V." 

Impulse  Wheel-,    benerattr       \  Transformer-'* 


'Concrete 


FIG.  296.  —  Sectional  elevation  of  the  Edison  Electric  Company's  hydro- 
electric station  at  Kern  River,  California,  containing  four  10,750  h.p.  Allis- 
Chalmers  impulse  wheels. 


375.  The  Keokuk,  Iowa,  Hydro -electric  Station  is  shown 
in  Figs.  300,  301  and  302.  The  ultimate  capacity  of  the  sta- 
tion is  300,000  h.p.  Energy  is  transmitted  over  a  distance  of 
approximately  200  miles  in  certain  directions  to  St.  Louis  and 
other  cities  in  Missouri  and  Iowa.  The  initial  installation  of 
turbines  comprises  fifteen  units  each  having  a  normal  rating 
of  10,000  h.p.  based  on  a  head  of  32  ft.  The  generators  have 
a  maximum  continuous  rating  of  9,000  kva.  at  11,000  volts 


SEC.  18] 


HYDRO-ELECTRIC  STATIONS 


319 


and  operating  at  80  per  cent,  power  factor.     The  normal  speed 
is  55.7  r.p.m.     This  is  a  25-cycle  plant.     For  energy 


36  Cycle. 
>.3?OVo/r,  3  Phase. 
Generator-^ 

\ 

I 


FIG.  297. — Sectional  elevation  of  the  31,000  h.p.  turbo  generator  units 
used  in  the  Yadkin  River  development  (in  North  Carolina)  of  the  Tallassee 
Power  Company.  (The  illustration  shows  the  method  used  in  dismantling 
the  runner.) 


mission  to  St.  Louis  the  voltage  is  stepped  up  to  110,000. 
For  shorter  distances  a  voltage  of  66,000  is  used. 


320 


CENTRAL  STATIONS 


[ART.  376 


376.  Outdoor  Hydro-electric  Plants*  have  been  proposed. 
Figs.  303,  304,  305  and  306  show  the  general  construction  and 
arrangement.  It  is  anticipated  that  material  economies 


FIG.  298. — Plan  view  of  the  Yadkin-River   hydro-electric   station. 


FIG.  299. — The  Yadkin-River  hydro-electric   development. 

will  be  realized  through  the  omission  of  the  building  (which 
ordinarily  houses  the  generating  equipment)  in  a  plant  of  this 
character.  Outdoor  switching  and  transforming  stations 

•  Electrical  Review,  Sept.  25,  1915;  p.  689. 


SEC.  18] 


HYDRO-ELECTRIC  STATIONS 


321 


have  been  in  successful  operation  for  a  number  of  years,  so 
there  appears  to  be  no  reason  why  an  outdoor  hydro-electric 


Conductor  Cable--. 


FIG.  300. — Sectional  elevation  of  the  Keokuk  hydro-electric  station. 

plant  should  not  also  be  successful.     The  plant  illustrated 
was  a  tentative  design  proposed  by  R.  J.  McClellan,  Chief 


322 


CENTRAL  STATIONS 


[ART.  376 


Engineer  of  the  Electric  Bond  and  Share  Company.  Cli- 
matic conditions  and  violent  winds  were  the  determining 
causes  for  eliminating  the  power-house  superstructure.  All 


Transformer;  -Gate  Room 


fvr'i+nf a     Q^^O  '-bBrt  ^     B  "^  ^B^B 

^/^-ja4-^^^£jg^^a--^a^-J^--^J 


''Auxiliary  '-Main 

Generator  Generator 


''•generator 
Pedestal 


*-£ 

^m , 

mJil 


FIG.  301. — Plan  view  of  half  of  the  Keokuk  hydro-electric  station.     (Com- 
pare this  with  the  sectional  elevation  shown  in  another  picture.) 


FIG.  302. — The  Keokuk  (Iowa)  development  of  the  Mississippi  River  Power 
Company. 

of  the  generators  and  transformers  are  located  outdoors. 
The  control  boards  and  exciters  are  installed  in  a  structure 
over  the  tail-race  where  a  repair  shop  also  is  located. 


SEC.  18] 


HYDRO-ELECTRIC  STATIONS 


323 


FIG.  303. — Showing  general   arrangement  of  the  proposed  outdoor  hydro- 
electric station. 


••Concrete  ,Hotor  (generator  Sefs 


(xtrrtiy 
Crane 


E- Sect  ion  Through  Transformer  Yard 

FIG.  304. — Showing  various  sections   taken   through   the    outdoor  hydro- 
electric station. 


324 


CENTRAL  STATIONS 


[ART.  376 


.Entrance  and      ,Ren'froaef  Tracks 

:  Trans.  Pit  \        Transformer  Yard 


umwww 


72Ft.->\ 


S«ts 


FIG.  305. — Plan  view  of  proposed  outdoor  hydro-electric  station. 


••SO  Ton  Cxmfry  Crane 


Concrete-' 
Rock 


Section  C-C 
Fio.  306. — Section  C-C.  Through  the  generator  platform. 


INDEX 


Aerial  circuits,  line  reactance,  141 
Air-blast  transformers,  276 
Allis-Chalmers  Co.,  318 
Alternating-current    circuits,    calculation 

and  design,  140-172 
determining  demand  factors,  43 
conversion  to  direct,  186 
demand  meter,  33 
enerators,  voltages  for,  268 


gen 
ligh 


tning  protectors,  table  of  applica- 
tion, 217,  218 
line,  measuring  demand,  27 
mechanical    remote    control    switch- 

boards, 254 
motor  installations,    demand  factors 

for,  50 

regulator,  222 
switchboards,  244 

for  three-phase  service,  248 
transmission  systems,  180 
voltages  and  applications,  183 
Alternating  voltages,   generation  of,   265 
Ammeter,  graphic,  28 

indicating,  26 

Amsler-type  planimeter,  83 
Annual  load  factor,  90 

plant  factor,  93 
Apparatus  for  lightning  protection,  195- 

218 

Arkansas  Light  and  Power  Co.,  301 
Atmospheric  lightning,  196 
Automatic   voltage   regulators,   219-228 


Barstow,  J.  S.,  298 
Boilers  for  steam  ger 

305 
Branch  circuit,  3 


iting  stations,  302, 


Calculation  of  alternating-current  circuits, 

140-172 

of  direct-current  circuits,  132-139 
Capacities  of  generators,  270-272 

load  and  plant 


Capacity  factor,  95 
disti 


inguished  fro 
factor,  96 
reserve,  271 

Carborundum  block  protector,  201 
Central    station,     determining   maximum 

demand  on,  64 
prime  movers,  281-286 
rates,  76 
See  also  Stations. 

Centralization  of  generating  stations,  264 
Charges   per    kilowatt-hour,  effect  of    de- 

creasing load  factor,  92 
Chicago,  load  graohs  for,  109 

report  of  load  factors,  etc.,  of  lighting 
customers,  86,  88 


protector, 


Choke  coils,  air  insulated,  216 
for  low-voltage  circuits,  215 
function,  199 
oil-insulated,  216 
selecting,  215 

Chuse-Poppet  valve  engine,  295 
Circuit-breaker  iype  li 

202 
Circuit  design,  118-131 

allowable   voltage   drop   in   incan- 

descent-lamp circuits,  120 
application  of  Ohm's  law,  118 
apportioning  voltage  drop  among 

components,  121 
basis  for  calculating  voltage  drop, 

conductors,  large  advisable.  120 
determining  loads  for  conductors, 

129 
distance  to  load  center  of  the  cir- 

cuit, 130 
drop  of  voltage  in  any  conductor, 

124 
finding  current  with  known   drop 

and  wire  length,  127 
formula  for  power  loss  in  a  conduc- 

tor, 128 

incandescent  lamp  circuits,  120 
length  of  circuit  for  known  current 

and  conductor,  128 
lighting  circuits,  119 

or     power     circuit,     computing 

voltage  drop,  125 
motor  circuits,  121 
apportionment      of 

122 
National   Electric   Code  rules   for 

wire  size,  130 
noting  ampere  loads  on  wiring  plan, 

130 
power  loss  in  a  direct-current  two- 

wire  circuit,  129 
in  conductor,  120 
principle  of  voltage  drop,  119 
resistance  of  copper  wire,  124 
safe-current-carrying    capacity    of 

wires,  table,  122,  123 
scale  for  measuring  plans,  131 
voltage  drop  and  size  of  wire,  118 
wire  size  for  known  circuit,  etc.,  127 

sizes,  118 

Circuit  reactance,  141 
Circuits,  alternating-current,  design,  140- 

172 
factors    affecting    computation, 

140 
inductive        interaction,        pre- 

venting, 142 
line  reactance,  140 
Mershon    diagram    for    voltage 

drop,  154 
polyphase    circuits,     conductors 

in,  142 
power  factors  of  apparatus,  140 


drop 


325 


326 


INDEX 


Circuits,  alternating-current,  design  re- 
actance, effect  on  wire-size 
formula,  147 

reducing  line  reactance,  141 
resistance  drop,  formula,  149 
single-phase   wire  size,  146,  149, 

symmetrical      arrangement      of 

wires,  145 
tables  for  finding  drop  in  lines, 

153,  155 
three-phase    circuit,    wire    size, 

165,  169 

transmission,  143 
three-wire,     three-phase     trans- 
mission  equal  to   two  single- 
phase,  169 
two-phase  transmission,  142 

wire  size,  160,  161,  163 
unbalancing  of  system,   145 
volts  loss,  determining,  148,  149 
wire     size,      determining     with 

Mershon  diagram,  156 
for  single-phase  branches  from 

three-phase  mains,  172 
for  single-phase  circuits,   146, 

149 

for  three-phase  circuit,  165, 169 
for  two-phase  circuits,  graphic 

method,  163 
for  two-phase     circuits,     with 

resistance,  164 
reactance  lacking,  147 
with  reactance,  149 
wires,    three-phase,   transposing, 

144 

Circuits,  branch,  3 
Circuits,  direct-current,  design,  132,  139 

balancing     three-wire     circuits, 

138,  139 

calculating  two- wire  circuits,  132 
conductor    sizes    for    three-wire 

circuits,  135 
conductors,  132 
three-wire  circuits,  135 
voltage       drop      in      three-wire 

circuit,  137 

Circuits,  distribution,  189 
feeder,  3 
multiple,  190 
polyphase,  142 
ring,  194 

single-phase,  see  Single-phase  circuits, 
tap,  6 

two-phase,  see  Two-phase  circuits. 
Commonwealth  Edison  Company,   110 
Compensator  for  line  drop,  228 
Comnression-type   protector,    210 
Condenser-type  lightning  protector,  202 
Conductors,    arrangement    in   polyphase 

circuits,  142 
spacing  of  National  Electrical  Code, 

141 

See  also  Wire. 
Connected  load  defined,  99 

use  in  computing  load  factors,  85 
Connected-load  factor,  73-100 
basis  of  computation,  97 
definition,  96 
equations,  98 

graph  of  energy  consumption,  99 
report  of  tests  in  Chicago,  86,  88 
Continuous  rating  of  apparatus,  94 
Contour  map  load  graph,  115 
Control-desk  switchboards,  239 
pedestals,  239 


Conversion    from    alternating    to    direct 

current,  186 
Copper  losses,  9 
Core  losses,  9 
Corliss  engines,  288,  290 
Cost  of  generating  electrical  energy,  259 
Cravath,  J.  R.,  90  _ 

Cross-compound  principle  in  turbines,  300 
Cupped-disc  gap  protector,  208 
Current,  computing,  for  circuits,  127 

D 

Dann,  W.  H.,  278 
Definitions  of  terms,  1-6 
Demand  factors,  application,  44 

definition,  42 

determination,  43 

for  lighting  installations,   47-49 

for  motor  installations,  49,  50 

importance     of,     in     determining 
transformer  capacities,  51 

of  alternating-current  and  direct- 
current  circuits,  43 

report  of  tests  in  Chicago,  86,  88 

tables,  46 

used  in  computing  load  factor,  85 
Demand,  maximum,  15-52 

alternating-current  line,  measuring 
demand,  27 

ammeter  for  measuring  demand,  20 

application  of  demand  factors,  44 

approximate    factors    for    lighting 
service,  47,  48 

average  demand,  15,  18 

definition  of  demand,  15 

determination  of  demand  factor,  43 

determining,  formulas,  64 

value  for  computing  load  factor, 

83 

with  an  ammeter,  26 
with  transformer,  27 

direct-current  line,  measuring   de- 
mand, 26 

examples,  time  interval  and  maxi- 
mum demand,  22 

factor,  demand,  42 

factors    for    lighting    installations, 
47^9 

importance     of,    in      determining 
transformer  capacities,  51 

integrating  graphic  meter,  38 
indicating  meters,  33 

maximum  demand,  definition,  16 

measuring  instruments,  24 

meters,  17,  21 
variation  of,  24 

methods  of  averaging  the  load,  23 

motor  installations,  factors  for,  49, 
50 

printometer-type  meters,  36 

tables  of  factors,  46 

thermal    or    thermostatic    meters, 
32 

time  interval,  20 
for  meters,  21 

unit,  17 

wattmeter,  28-31 

Westinghouse  R.  O.  demand  meter, 

Westinghouse       recording-demand 

watt-hour  meter,  41 
Wright  meters,  31,  48 
Demand-measuring   instruments,    24 

rnpters,  see  Meters,   demand. 
Demands,  diversity  among,  60,  61,  62,  63 


INDEX 


327 


Department  store  plant,  load  graph,  111 
Depreciation  of  equipment  of  an  electric 

plant,  263 
Design     of    alternating-current     circuits, 

140-172 

of  circuits,  118-131 
of  direct-current  circuits,  132-139 
of  hydro-electric  stations,  315 
Designing  plants,  importance  of  diversity. 

Diagram,  Mershon,  see  Mershon  diagram. 
Direct-current    circuits,  determining  de- 
mand factors,  43 
conversion  from  alternating,  186 
demand  meter,  35 
for    transmission    and    distribution, 

177 
generators,     voltage    regulators    for, 

221,  225 

line,  measuring  demand,  26 
motor  installations,   demand  factors 

for,  50 

switchboards,  239-243 
systems,  in  generating  stations,  265 
voltages,  in  generating  stations,   265 
Distributing  center,  6 
system,  definition,  2 

diversity  factors  for,  62 
Distribution  circuits,  189 
parallel,  190 
series,  189 

Distribution  loss  factors,  7-14 
approximate,  14 
factors,  12 
leakage  loss,  9 
line  loss,  8 

factor,  13 
meter  losses,  9 
numerical  illustration,  11 
probable  factors,  13 
stolen-ene  gy  loss,  10 
transformer  loss,  9 
types,  8 

Distribution  of  electrical  energy,  173-194 
Distribution-system  terms,  1-6 
branch  circuit,  3 
distributing  center,  6 

system,  2 
feeder  circuit,  3 
main,  3 
service,  3 
sub-feeder,  3 
sub-main,  3 
tap  circuit,  6 
tie  line,  1 
transmission  line,  1 

system,  2 
Diversity  and  diversity  factors,  53-72 

central-station  distributing  system, 

62 
commercial      lighting     consumers, 

demands,  62 
definition,  53 
determining,  59 

kilowatt  station  capacity,  71 
different  factors  among  components 

of  a  system,  60 
diversity  factor  defined,  56 
effect  of  increasing,  on  load  factor, 

75 
eliminating  apparatus  by  grouping 

consumers,  71 

factor  used  in  computing  load  fac- 
tor, 85 

factors  for  a  central  station  distrib- 
uting system,  62 


Diversity  and  diversity  factors,  feeders, 
diversity  of  demands  among,  70 

formulas  for  determining  maximum 
demand,  64 

llustration  of  diversity  of  demand, 
55 

importance  in  plant  design,  72 

lighting  transformers,  demands  on 
mains  by,  63 

residence-lighting  consumers,  de- 
mands, 61 

sub-stations,  diversity  of  demands, 
70 

total  diversity  factor  for  a  system, 
70 

values  determined  by  local  condi- 
tions, 61 

E 

Efficiency  of  transmission,  175 

formula,  176 

Electric  Bond  and  Share  Co.,  322 
Electric  generating  stations,  259-280 

advantages    of    centralization    of 

plant,  264 

capacities    and    ratings    of  gener- 
ators, 270-272 
cost  per  unit  of  energy,  259 
depreciation  of  equipment,  263 
direct-current  voltages  and  systems, 

265 

exciters,  273 
external  reactances,  278 
factors     determining    location    of 

apparatus,  272 
generation  of  alternating  voltages, 

265 
grounded  and  ungrounded-neutral 

systems,  267 
locating  exciters,  274 
location,  264 
percentage  reactance  of  a  reactor, 

279 
power  factor,  effect  on  capacity  of 

generator,  272 
prime-movers,  location,  273 
reactors,  278 

single-phase  transformers,  274,  275 
star-connected  generators,  266 
switchboard,  location  of,  279 
system  of  excitation,  273 
three-phase   alternating-c  u  r  r  e  n  t 

systems,  265 
transformers,  274,  275 
three-wire  systems,  265 
transformers,  274-277 
two-wire  systems,  265 
unit  principle  of  installation,  272 
voltage  regulators,  274 
voltages  for  alternating-c  u  r  r  e  n  t 

generators,  268 
Electric    Power    Club,    standard   voltage 

ratings,  179,  183 
railways,  see  Railways,  electric. 
Electrical  energy,  see  Energy,  electrical. 

remote-control  switchboards,  257 
Electrolytic  lightning  protector,  210-214 
Electromagnetic  inductance,  141 
El  well,  David,  300 
Energy,  electrical,  computing,  for  a  given 

installation,  91 

consumption  in  kilowatt  hours,  99 
cost  of  generating  per  unit,  259 

transmission  and  distribution,  173- 
194 


328 


INDEX 


Energy,  electrical,  wire  sizes  for  distribu- 
tion, 118 
Engines,  internal-combustion,  306 

steam,  298 

uniflow,  291-295 
Exciters  in  generating  stations,  273 

locating,  274 

voltage,  274 
External  reactances  in  generating  stations, 


Factor,  capacity,  95 

connected-load,  96 

distinction  between  plant,  load,  and 
capacity,  96 

distribution-loss,  12 

diversity,  53-72 

plant,  92 
Feeder-and-main  circuit,  192-194 

circuit,  definition,  3 

regulator,  186 
Feeders,   determining  maximum  demand 

on,  66 

Fernald,  R.  H.,  284 
Frequencies,  data  for  U.  S.,  179 
Frequency-changer  sub-station,  189 

standard,  184 


Gear,  H.  B.,  59,  75 

General    Electric    Company,     demand 

meters,  33,  36,  38 

Generating  stations,  electrical,  259-280 
hydro-electric,  310-324 
internal-combustion-engine,      306- 

309 

steam,  287-305 

Generation  of  alternating  voltages,  265 
Generators,   alternating-current,   voltages 

f9r,  268 

capacities  and  ratings,  270-272 
direct-current,  voltage  regulators  for, 

221,  225 

Goedjen,  A.  J.,  179 

Graded-shunt    resistance    protector,    205 
Graphic  wattmeter,  28 
Graphs,  load,  101-117 
Gravity  wheel,  310-313 
Grounded-neutral  systems,  in  generating 
stations,  267 


Hackett,  H.  Berkeley,  260 
Hagenlocher,  E.,  293 
Hall,  C.  I.,  24 
Horn-gap  protectors,  208 
Hotel  plant,  load  graph,  111] 
Hyde,  T.  B.,  263 
Hydraulic  prime  movers,  285 
turbine,  largest  built,  317 
Hydro-electric  stations,  286,  310-324 

applications    for    types    of    water- 
wheels,  313 

arrangement    of   low-head    equip- 
ment, 315 

classes,  310 

design  of,  315 

development  of  low-head,  286 

efficiency  of  water  wheel,  312 

gravity  wheel,  310,  313 

high-head  station,  316 

horizontal    turbine   in    low-head 
plant,  316 


Hydro-electric    stations,   impulse    wheel, 

310,  313 

Keokuk,  Iowa,  plant,  318 
largest  single-runner  turbine,  317 
nomenclature,  313 
outdoor  plants,  320 
reaction  wheel,  310,  313 
waterwheels,  310 

I 

mpulse  wheel,  310,  313 
ncandescent-lamp  circuits,  voltage  drop 

ndicating  ammeter,  26 

nductance,  electromagnetic,  140 

nductiye  interaction,  preventing,  142 

ndustnal  plant,  load  graph,  106 

nstruments  for  measuring  demand  24 

nsull.  Samuel,  282,  283 

ntegrating    graphic   demand    meter,    38 

nteraction,  inductive,  142 

nterborough  Rapid  Transit  Co.  of  N.  Y., 

300 

nterior-wiring  system,  terms,  6 
nternal-combustion-engine  prime  movers, 

stations,  306-309 

efficiency  of  engines,  306 
lay-out     of     small     oil-engine 

plant,  309 

oil  engines  for  small  plants,  308 
Internal  lightning,  197 
Interurban  street  railways,  load  graph,  107 

K 

Keokuk,  Iowa,  hydroelectric  station,   318 
Kilowatt  station  capacity,  determining,  71 


Leakage  loss,  9 

Lighting     circuits,    direct-current    trans- 
mission for,  177 
voltage  drop  in,  119-121,  125 
voltages  for,  179 
wire  sizes  for,  146, 149 
consumers,   diversity  of  demand  of 

61,  62 
installations,  demand  factors  for,  47- 

49 
plants,  load  graph,  105 

oil  engines  for,  308 
transformers,     diversity    among    de- 
mands on  mains,  63 
Lightning,  atmospheric,  196 
definition,  195 
explanation  of  paths,  205 
internal,  197 

Lightning  protection  apparatus,   195-218 
alternating-  and  direct-current  pro- 
tectors, 199 
alternating-current,  application  of, 

table,  217,  218 
atmospheric  lightning,  196 
carborundum  block  protector,  201 
choke  coils,  air-insulated,  216 
for  low-voltage  circuits,  215 
function,  199 
oil-insulated,  216 
selecting,  215 
circuit-breaker  type,  202 
combination  choke  coil  and  horn- 
gap  protector,  210 
compression-type,  210 


INDEX 


329 


Lightning  protection  apparatus,  condenser 
type,  202 


cupped-disc  gap  protector,  208 
'   "nition  of  lightning,  195 
of  protector,  197 


definitior 


direct  stroke  denned,  196 
distinction  from  lightning  arrester, 

195 

electrolytic,  210-214 
explanation  of  paths  of  lightning, 

205 
graded-shunt  resistance  protector, 

205 

horn-gap  protectors,  208 
induced  stroke,  196 
internal  lightning,  197 
magnetic    blow-out,  direct-current 

protector,  200 
non-arcing  metal  caps,  205 

cylinder  protectors,  204 

on  electric  railway  cars,  201 

principle  of  protector,  197 

selecting  choke  coils,  215 

Line  drop,  compensation  for,  in  voltage 

regulators,  227 
loss,  8 

factor,  13 

in  commercial  series  circuits,  190 
reactance,  140 
Lloyd,  E.  W.,  86 
Load  curve,  80 
Load  factor,  73-100 

annual,  or  yearly,  equation,  90 
computing   energy  delivered   by   a 

given  installation,  91 
decrease,  effect  on  charges,  92 
definition,  73 

determining    m  a  x  i  m  u  m-demand 
value,  83 

power  from  a  load  curve,  80 
distinguished   from   plant    and  ca- 

pacity factor,  96 

ect  of  addition  of  off-peak  loads, 
111 

of  increased  diversity  of  demand, 
75 

on  central-station  rates,  76 
equations  for  computing,  85,  90 
formulas,  73 
formulas   for    average   power   con- 

sumption, 78-83 
operating,  equation,  90 
period  for  reckoning,  78 
polar  planimeter,  use  of,  81-83 
report  of  tests  in  Chicago,  86,  88 
significance,  74 
Load  graphs,  101-117 

adding  to  obtain  resultant,  116 
addition    of   off-peak   loads,    effect 

on  load  factor,  111 
annual,  114 

characteristic  for  small  towns,  112 
city  street  railway,  106 
combined  lighting,  industrial,  and 

railway  loads,  .108,  109 
comparison  of  winter  and  summer, 

112 

contour  map,  115 
department  store  plant,  111 
hotel  plant,  111 
industrial  load,  typical,  106 
interurban  street  railways,  107 
large  cities,  109 
obtaining  data,  102 
office  building  plant,  110 
period  of  time,  104 


p 
effec 


Load  graphs,  significance,  103 

storage    battery    used    to    modify 

load  demands,  117 
typical  electric-lighting  load,  105 
unit  for  ordinate  values,  104 
variation   with   different   types   of 

load,  104 
Location  of  a  generating  station,  264 

of  apparatus  in  generating  stations, 

272 

Lof,  E.  A.,  266,  269,  273,  275,  281,  296 
Loss,  distribution,  7-14 

M 

Main,  definition,  3 
Maximum  demand,  15-52 

rating,  94 

McClellan,  R.  J.,  321 
Mershon  diagram,  determining   wire  size 

with,  156 

for  three-phase-circuit  wire,  169 
for  two-phase  circuit  wire,  164 
for  voltage  drop,  154 
Mershon,  Ralph,  142 

Metal  cylinder  lightning  protectors,   204 
Meter  losses,  9 
Meters,  demand,  17,  21 

integrating  graphic,  38 
integrating  indicating,  33 
printometer-type,  36 
variations  of  rating,  24 
Westinghouse  RO,  28-31 
Westinghouse       recording-demand 

watt-hour,  41 
Wright,  31,  48 

Meters,  thermal  or  thermostatic,  32 
Motor  branch  circuits,  wire  size,  122,  130, 

148 
circuits,    apportionment    of    voltage 

drop,  122 

voltage  drop  in,  121 
-generator  substation,  187 
installations,  demand  factors  for,  49, 

50 
Multiple  circuits,  190 

N 

National   Electric  Code  rules,  for  motor 

branch  circuit,  122,  130,  148 
spacing  of  conductors,  141 
wire  size,  133,  134 
National  Electric  Light  Association,  86 

standard  voltages,  183 
New    England    National    Electric    Light 

Association,  300 

New  York  Edison  Company,  110 
Interborough  R.  T.  Co.,  300 
load  graphs  for,  109 
Nilsen,  P.  J.,  64 


Office  building  plant,  load  graph,  110 
Ohm's  law,  applied  to  circuit  design,  118 

computing  voltage  drop  by,  124,  174 
Oil-cooled  transformers,  276 

engines,  308 
Operating  load  factor,  90 

plant  factor,  93 
Outdoor  hydro-electric  plants,  320 


Panel  box,  6 

switchboard,  231 


330 


INDEX 


Parallel  distributing  circuits,  190 
Pedestal  switchboards,  239 
Pelton  wheel,  316 
Pigott,  R.  J.  S.,  297 
Planimeter,  polar,  81-83 
Plant  factor,  73-100 

annual,  93 

compared  to  capacity  factor,  95 

continuous  rating,  94     - 

definition,  92 

distinguished  from  load  and  capac- 
ity factor,  96 

high,  95 

operating,  93 
Polar  planimeter,  81-83 
Polyphase   circuits,   arrangement   of  con- 
ductors, 142 
Post  switchboards,  239 
Potential  regulator,  186 
Power   circuit,    computing    voltage    drop, 

computing  consumption,  78-83 
factor,  effect  on  capacity  of  a  gener- 
ator, 272 

factors  of  apparatus,  140 
loss  in  direct-current  two- wire  circuit, 

129 

in  transmission  of  energy,  173 
Prime  movers,  281-286 

advantages     of     turbo-generators, 

284 

hydraulic,  285 

internal-combustion  engine,  284 
low-head  hydro-electric  plants,  286 
steam,  281 

uniflow  engines,  291-295 
Printometer  indicators,  21 

-type  demand  meters,  36 
Protector  apparatus,  lightning,  195-218 
choke  coil  and  horn-gap,  210 
compression-type,  210 
cupped-disc  gap,  208 
electrolytic,  210-214 
graded-shunt  resistance,  205 
Protectors,  horn-gap,  208 
Public  Service  Corporation  of  X.  J.,  302 


Railways,  electric,  lightning  protector  on 

cars,  201 

load  graph  for,  106   107,  109 
switchboards  for,  243 
voltages  for,  179 
Rates  of  central  station,  76 
Rating,  continuous,  94 
Ratings  of  generators,  270,  271 
Reactance,    determining    wire    size    with 

Mershon  diagram,  156 
effect  on  wire  size  formulr,  147 
line,  140 

Reaction  wheel,  310,  313 
Reactors  in  generating  stations,  278 

percentage  reactance,  279 
Reciprocating  steam  engines,  consumption 

table.  290 

Regulators,  voltage,  219-228 
Remington   Arms   and  Ammunition   Co., 

302 
Remote-control    switchboards,    electrical, 

257 

mechanical,  253 

Reserve  capacity  of  generators,  271 
Resistance  drop,  formula  for  finding,  149 
of  copper  wire,  124 
protector,  graded-shunt,  205 


Rhodes,  G.  I.,  95,  101,  105 
Ring  circuit,  194 
Rotary  converter  substation,  186 
Rudd,  H.  H.,  278 


Salmon,  F.  W.,  289 

Sanderson,  C.  H.,  252 

Scale  for  measuring  plans,  131 

Series  distributing  circuits,  189 

Service,  definition,  3 

Shuster,  J.  M.,  271,  287 

Single-phase   alternating-current   circuits, 

wire  size  for,  146, 

calculation  by  graphic  method,  149 
determining    with     Mershon    dia- 
gram, 156 
circuits,  two  can  replace  a  three- wire 

three-phase  transmission,    169 
transformers,  274,  275 
Star:connected  generators,  266 


Station  capacity,  kilowatt,  71 
iions,  electric  generating, 
hydro-electric,  310-324 


Stations,  electric  generating,  259—280 


internal  combustion-engine,  306-309 
steam  generating,  287-305 

turbine,  296 

See  also  Central  stations. 
Steam  engine,  compared  with  steam  tur- 
bine, 298 
Steam  generating  stations,  287-305 

alternating-current  generator  with 

slide-valve  engine,  291 
applications  of  steam  turbine  and 

steam  engine,  298 
belted  vs.   direct-connected   units, 

288 

boilers,  302,  305 
capacities  of  turbo-generator  units, 

classes,  287 

design    for   turbo    plant    with    one 

.unit,  302 
direct-connected    generating    unit. 

290 

installation  of  uniflow  engine,  295 
large  turbo-generator  plant,  302 
medium-sized  turbo  plant,  301,  302 
small  plants,  287 

steam  consumptions  of  reciprocat- 
ing engines,  table,  290 
of  turbines,  300 
steam  turbines,  296 
uniflow  engines,  291-295 
Steam  prime  movers,  282 
Steam  turbines,   capacities   of  turbo-gen- 
erator units,  297 
compared  with  steam  engine,  298 
cross-compound  principle,  300 
economy,  296 
efficiency  graph,  297 
stations,  296 
steam  consumptions,  300 
Stolen-energy  loss,  10 
Storage    battery    used    to    medify    load 

demands,  117 
Stott,  H.  G.,  282 
Street    railways,    lightning    protector    for 

cars,  201 

load  graph  for,  106,  107,  109 
switchboard  for  direct-current  ser- 
vice, 243 
voltages  f9r,  179 
Sub-feeder,  definition,  3 
Sub-main,  definition,  3 


INDEX 


331 


Sub -stations,  184 

frequency  changer,  189 
function  of  equipment,  185 
motor-generator,  187 
synchronous  or  rotary  converter,  186 
transformer,  185 

Switchboards  and  switch-gear,  229-258 
advantages    of    remote-control    over 
self-contained  boards,  252 

alternating-current,  244 

alternating-c  u  r  r  e  n  t    mechanical 
remote-control  board,  253 

alternating-current      switchboards 
for  three-phase  service,  248 

arranging  the  panels,  233 

circuit  diagrams  for  direct-current 
switchboards,  240 

classes,  231 

control-desk  switchboards,  239 

diagram,  233 

direct-current,  239-243 

direct-current  railway  service,  243 

distinction  between,  229 

electrical  remote-control,  257 

fittings  for  supporting  switchgear, 
236 

frames  for  panel  switchboards,  235 

function,  229 

laying  out,  231 

location  in  station,  279 

material  for  panel  sections,  237 

panel  switchboards,  231,  238 

pedestal  switchboards,  239 

post  switchboards,  239 

proportions  of  panels  and  sections, 

remote-control,  230 

alternating-current  boards,  245 
self-contained,  230 
three-wire    direct-current    switch- 
boards, 243 

two-wire      direct-current     switch- 
boards, 241,  242 
Synchronous  sub-station,  186 
Systems,  data  for  U.  S.,  179 
direct-current,  265 
grounded    and    ungrounded-neutral, 

267 

three-phase,  212 
three-wire.  265 
transmission,  2 


Tsp  circuit.  6 
Thermal  meters,  32 
Thermostatic  indicator,  21 

meters,  32 

Thompson,  W.  R.,  315 
Three-phase  alternating-current  systems, 

circuit,   wire  size,   with  and   without 

reactance,  165,  169 
generators,  star-connected,  266 
service     alternating-current    switch- 

boards. 248 
systems,  arrangement  of  electrolytic 

protectors,  212 
transformers,  274,  275 
transmission,  143 

replaced    by   single-phase    circuits, 
169 

transposing  wires,  144 
Three-  wire  circuit,  balancing,  138,  139 
systems  in  generating  stations,  265 


systems  n  genera 
Tie  line,  definition,  1 


Time  interval  for  determining  maximum 

demand,  20 

Thrill  voltage  regulator,  220 
Transformer  capacities,  determining,  51 
losses,  9 

sub-station,  185 

Transformers,  determining  maximum  de- 
mand on,  64 

for  measuring  demand  in  alternating- 
current  lines.  27 
in  generating  stations,  274-277 
lighting,  diversity  among  demands  on 

mains,  63 

oil-cooled,     water-cooled,     and     air- 
blast.  276 

Transmission  line,  definition,  1 
Transmission  of  electrical  energy,  173-194 
alternating-current  systems,  180 
direct-current,    with   multiple  cir- 
cuits, 177 

distribution  circuits,  189 
efficiency  formula,  176 
efficiency  of  transmission  of  a  cir- 
cuit, 175 

feeder-and-main  system,  192-194 
for  lighting,  177 
for  power  motors,  177 
frequency-changer  substation,  189 
high  voltage  desirable,  173 
interior  systems,  193 
line  loss  in  commercial  series  cir- 
cuits, 190 

motor-generator  sub-station,  187 
out-of-door  circuit,  193 
power  lost  related  to  voltage,  1 73 
reasons  for  using  electricity,  173 
ring  circuit,  194 

rotary  converter  sub-station,  186 
series  distributing  circuits,  189 
standard   alternating-current   vol- 
tages, table,  183 
standard   direct-current    voltages, 

table,  179 

standard  frequency  in  U.  S.,  184 
sub-stations,  184 
synchronous  sub-station,  186 
three-phase  preferable,  182 
three- wire  distribution,  178 
transformer  sub-station,  185 
voltage  for  short  transmission,  175 
voltage  per  mile,  rule,  184 
weight  of  conductor  related  to  vol- 
tage, 175 

Transmission  system,  2 
Transposition  of  three-phase  circuits,  145 
Turbine,  hydraulic,  317 
Turbines,  see  Steam  turbines. 
Turbo-generator  stations,  301,  3C2 
Turbo-generators,  advantages  of,  284 
Two-phase     alternating-current    circuit*, 

determining  wire  size,  160,  161 
circuits,  graphic  method  of  deter- 
mining wire  size,  163 
wire  size  determined  by  Merslion 

diagram,  164 
transmission,  142 

U 

Unbalancing  of  a  system,  145 
Ungrounded-neutral   systems  in  generat- 
ing stations,  267 
Uniflow  engines,  291-295 
Unit,  for  values  of  a  load  graph,  104 

of  connected  load  and  power  input,  96 
of  continuous  rating  of  apparatus,  94 
of  maximum  demand,  17 


332 


INDEX 


Unit  principle  of  installation  of  plants,  272 
United  Electric  Light  and   Power  Com- 
pany, HO 


Vector  diagram  to  determine  volts  loss, 

148,  149 
Voltage  drop,  and  wire  size,  118 

basis  of  calculations,  124 

computing  by  Ohm's  law,  124 

diminishing,  142 

in  incandescent-lamp  circuits,  119- 
121 

in  lighting  or  power  circuit,  125 

in  motor  circuits,  121,  122 

in  three-wire  circuit,  137 

Mershon  diagram,  154 

principle  of,  119 

table  for  alternating-current   lines 
with  Mershon  diagram,  153,  155 
Voltage  regulators,  219-228 

alternating-current  regulator,  prin- 
ciple, 222 

arrangement  of,  for  alternators,  223 

capacity  of  relay  contacts,  225 

compensation  for  line  drop,  227 

connections  for  different  services, 
228 

factors  causing  variations  in  vol- 


rrent  generators,  221, 


tage,  219 
for  direct-cu 


225 


function    of    automatic    regulator, 

220 

in  generating  stations,  274 
installing,  228 
operation  of,  in  parallel,  226 


principle,  220 
Tirrill  tj 


type,  220 

Voltages,  alternating,  generation  of,  265 
data  in  U.  S.,  179 
for  alternating-current  generators,  268 


Voltages  of  exciters,  274 

standard      alternating-current,      and. 

their  applications,  179 
table,  183 
standard    direct-current,    and    their 

applications,  179 
Volts  loss,  determining,  148.  149 


Water-cooled  transformers,  276 
Waterwheels,  310 

applications  for  types,  313 

efficiency,  312 
Wattmeter,  graphic,  28 
Westinghouse     direct-current    generator. 

Electric    &   Manufacturing   Co.,    142 
RO  demand  meter,  28-31 
recording-demand    watt-hour    meter, 

41 
Wire  sizes,  calculating,  146,  149 

determining     with     Mershon     dia- 
gram, 156 

finding,  for  known  current,  etc.,  127 
for  distribution  of  electric  energy, 

118 
for     single-phase     branches     from 

three-phase  mains,  172 
for   three-phase    circuit    with    and 

without  reactance,  165,  169 
for  two-phase  circuits,  160,  161 

with  resistance,  164 
graphic  method  for  two-phase  cir- 
cuits, 163 
Wires,      safe-current-carrying      capacity, 

table,  122,  123 
Woods,  3.  B.,  301 
Wright  demand  meter,  22,  31,  48 


Yadkin  River  plant,  317 
Yearly  load  factor,  90 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 

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1191  Central  stations 


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